package contest210416; import java.io.*; import java.util.ArrayDeque; import java.util.Arrays; import java.util.InputMismatchException; import java.util.Queue; import java.util.function.IntUnaryOperator; import java.util.function.LongUnaryOperator; public class H2 { InputStream is; FastWriter out; String INPUT = ""; static final int mod = 998244353; static long K, L; public void solve() { int n = ni(); K = ni(); int[] from = new int[n]; int[] to = new int[n]; for (int i = 0; i < n; i++) { from[i] = ni() - 1; to[i] = ni() - 1; } int[][] g = packU(n, from, to); AnalyzeResult ar = analyze(g); L = invl(mod+1-K, mod); // cycle[0]-cycle[-1]を切った木についてまず解く int[][] h = makeTree(g, ar); long ans = solveForTree(h); // 残ったケースはかならずcycle[0]-cycle[-1]を含む。 // - cycleのすべての辺を含む場合 cycleの要素までのDPの結果に、一度にcycleの辺を全部追加する。 // - それ以外の場合、cycleのどの辺で切るかで値を列挙しておき、両方向をO(|cycle|)でマージする。 // 足についてDP Datum[] dp = solveForSideTrees(ar, g); // 全辺追加。cycleの要素はすでに次数2以上の扱いをされている Datum base = new Datum(1, 0, 1); for(int c : ar.cycle){ base = merge(base, dp[c]); } ans += base.f0; Datum d11 = new Datum(2L*K*L % mod, K*L % mod, K*L%mod); Datum d01 = new Datum(2L*L%mod, L, L); Datum[] pref = new Datum[ar.cycle.length]; Datum[] pre = new Datum[ar.cycle.length]; for(int i = 0;i < ar.cycle.length;i++){ int cur = ar.cycle[i]; pref[i] = i == 0 ? new Datum(dp[cur].f1, dp[cur].f0, dp[cur].fp) : merge(pref[i-1], dp[cur]); // curだけは次数1であることを許される。このとき次数2扱いしていたものを除く pre[i] = add(pref[i], i == 0 ? sub(d11, d01) : merge(pref[i-1], sub(d11, d01))); } Datum[] suff = new Datum[ar.cycle.length]; Datum[] suf = new Datum[ar.cycle.length]; for(int i = 0;i < ar.cycle.length;i++){ int cur = ar.cycle[ar.cycle.length-1-i]; suff[i] = i == 0 ? new Datum(dp[cur].f1, dp[cur].f0, dp[cur].fp) : merge(suff[i-1], dp[cur]); // curだけは次数1であることを許される。このとき次数2扱いしていたものを除く suf[i] = add(suff[i], i == 0 ? sub(d11, d01) : merge(suff[i-1], sub(d11, d01))); } // tr(ar.cycle); // tr(pre); // tr(suf); Datum sufcum = new Datum(0, 0, 0); for(int i = ar.cycle.length-2;i >= 0;i--){ sufcum = add(sufcum, suf[ar.cycle.length-2-i]); Datum merged = merge(pre[i], sufcum); ans += merged.f0; } ans = ans % mod * pow(mod+1-K, n, mod); out.println(ans % mod); } Datum[] solveForSideTrees(AnalyzeResult ar, int[][] g) { int n = g.length; Datum d01 = new Datum(2L*L%mod, L, L); // Datum d00 = new Datum(1L, 0L, 1L); Datum d11 = new Datum(2L*K*L % mod, K*L % mod, K*L%mod); Datum[] dp = new Datum[n]; // treeと異なり上に継承するデータのみでよい for(int i = n-1;i >= 0;i--){ int cur = ar.ord[i]; Datum base = d01; for(int e : g[cur]){ if(ar.par[cur] == e)continue; if(ar.next[e] != -1)continue; base = merge(base, promote(dp[e])); } // +点ではじまるやつ(1,1) // -無に2ではじまるやつ (0,1) // base = base + d11 - d01 if(ar.next[cur] == -1) { base = sub(add(base, d11), d01); }else{ // cycle上のときは2ではじまるやつしかないので調整不要 } dp[cur] = base; } return dp; } long solveForTree(int[][] h) { int n = h.length; int[][] pars = parents(h, 0); int[] par = pars[0], ord = pars[1], dep = pars[2]; Datum d01 = new Datum(2L*L%mod, L, L); Datum d00 = new Datum(1L, 0L, 1L); Datum d11 = new Datum(2L*K*L % mod, K*L % mod, K*L%mod); Datum[] dp = new Datum[n]; long ans = 0; for(int i = n-1;i >= 0;i--){ int cur = ord[i]; Datum base = d01; Datum sum = d00; for(int e : h[cur]){ if(par[cur] == e)continue; base = merge(base, promote(dp[e])); sum = add(sum, dp[e]); } Datum minus = merge(sum, d01); Datum plus = merge(sum, d11); ans += base.f0; // tr(cur, base, minus, plus); // +点ではじまるやつ(1,1) // 無に2ではじまるやつを引く (0,1) base = sub(add(base, d11), d01); dp[cur] = base; // n(012) ans -= minus.f0; ans += plus.f0; ans += mod; // tr(ans%mod); } return ans%mod; } public static class AnalyzeResult { public int[] par, next, ord, cycle, icycle, dep, roots; } public static AnalyzeResult analyze(int[][] g) { AnalyzeResult ret = new AnalyzeResult(); int n = g.length; int[][] pn = split(g); int[] par = pn[0], next = pn[1]; ret.par = par; ret.next = next; int[] ord = makeOrder(g, ret.par); ret.ord = ord; int[] cycle = cycle(g, ret.next); int[] icycle = new int[n]; Arrays.fill(icycle, -1); for(int i = 0;i < cycle.length;i++)icycle[cycle[i]] = i; ret.cycle = cycle; ret.icycle = icycle; int[] depn = new int[n]; int[] roots = new int[n]; for(int i = 0;i < n;i++){ int cur = ord[i]; if(next[cur] != -1){ roots[cur] = cur; depn[cur] = 0; }else{ roots[cur] = roots[par[cur]]; depn[cur] = depn[par[cur]] + 1; } } ret.dep = depn; ret.roots = roots; return ret; } public static class AnalyzeResultEx extends AnalyzeResult { public int[][] tspar; public int[] tdep; } public static int[][] makeTree(int[][] g, AnalyzeResult res) { int n = g.length; int[][] tree = new int[n][]; int ngf = res.cycle[res.cycle.length-1], ngt = res.cycle[0]; for(int i = 0;i < n;i++){ int[] row = Arrays.copyOf(g[i], g[i].length); if(i == ngf || i == ngt){ int p = 0; boolean processed = false; for(int j = 0;j < row.length;j++){ if(row[j] == (ngf^ngt^i) && !processed || row[j] == i){ processed = true; }else{ row[p++] = row[j]; } } row = Arrays.copyOf(row, p); } tree[i] = row; } return tree; } /** * gから親リンクとringの次のリンクを取り出す。ringを含んでいなくても可能。森も取り出す。 * 重複辺は事前に除いておかないといけない。 * 重複辺を含んでいても大丈夫だとおもう * par!=-1,next==-1 : 木部ノード * par==-1,next!=-1 : 環部ノード * par==-1,next==-1 : 森のルート * @param g 重複辺を除いたpseudoforest * @return {par, next} */ public static int[][] split(int[][] g) { int n = g.length; int[] deg = new int[n]; int[] par = new int[n]; Arrays.fill(par, -1); for(int i = 0;i < n;i++){ deg[i] = g[i].length; } Queue q = new ArrayDeque(); for(int i = 0;i < n;i++){ if(deg[i] == 1){ q.add(i); } } while(!q.isEmpty()){ int cur = q.poll(); deg[cur] = -9999999; for(int e : g[cur]){ deg[e]--; if(deg[e] >= 0){ par[cur] = e; } if(deg[e] >= 0 && deg[e] <= 1){ q.add(e); } } } boolean[] ved = new boolean[n]; int[] next = new int[n]; Arrays.fill(next, -1); for(int i = 0;i < n;i++){ if(!ved[i] && deg[i] == 2){ ved[i] = true; int cur = i; outer: while(true){ for(int e : g[cur]){ if(deg[e] == 2 && !ved[e]){ next[cur] = e; ved[e] = true; cur = e; continue outer; } } next[cur] = i; break; } } } return new int[][]{par, next}; } /** * cycle側が先頭にくる * @param g * @param par * @return */ public static int[] makeOrder(int[][] g, int[] par) { int n = g.length; int[] ord = new int[n]; int p = 0; boolean[] ved = new boolean[n]; for(int i = 0;i < n;i++){ if(par[i] == -1){ ord[p++] = i; ved[i] = true; } } for(int r = 0;r < n;r++){ for(int e : g[ord[r]]){ if(!ved[e]){ ved[e] = true; ord[p++] = e; } } } return ord; } // cycle部分を取り出す public static int[] cycle(int[][] g, int[] next) { int n = g.length; int[] which = new int[n]; int p = 0; for(int i = 0;i < n;i++){ if(next[i] != -1){ int u = i; while(true){ which[p++] = u; u = next[u]; if(u == i)break; } break; } } return Arrays.copyOf(which, p); } public static int[][] parents(int[][] g, int[] clus) { int n = g.length; int[] par = new int[n]; Arrays.fill(par, -1); int[] q = new int[n]; int[] depth = new int[n]; int p = 0; for(int i = 0;i < n;i++){ if(clus[i] == 0){ int root = i; q[p] = root; depth[root] = 0; par[root] = -1; for(int r = p++;r < p;r++){ int cur = q[r]; for(int nex : g[cur]){ if(par[cur] != nex && clus[nex] != 0){ q[p++] = nex; par[nex] = cur; depth[nex] = depth[cur] + 1; } } } } } return new int[][] {par, q, depth}; } public static long pow(long a, long n, long mod) { // a %= mod; long ret = 1; int x = 63 - Long.numberOfLeadingZeros(n); for (; x >= 0; x--) { ret = ret * ret % mod; if (n << 63 - x < 0) ret = ret * a % mod; } return ret; } static class Datum { long f1; // (y+1)K^xL^y long f0; // yK^xL^y long fp; // K^xL^y public Datum(long f1, long f0, long fp) { this.f1 = f1; this.f0 = f0; this.fp = fp; } @Override public String toString() { return "Datum{" + "f1=" + f1 + ", f0=" + f0 + ", fp=" + fp + '}'; } } public static Datum merge(Datum A, Datum B) { long nfp = A.fp * B.fp % mod; long nf0 = ((A.f1 * B.f1 - A.f0 * B.f0 - nfp) % mod + mod) % mod; long nf1 = ((A.f1 * B.f1 - A.f0 * B.f0) % mod + mod) % mod; return new Datum(nf1, nf0, nfp); } public static Datum add(Datum A, Datum B) { long nfp = (A.fp + B.fp) % mod; long nf0 = (A.f0 + B.f0) % mod; long nf1 = (A.f1 + B.f1) % mod; return new Datum(nf1, nf0, nfp); } public static Datum sub(Datum A, Datum B) { long nfp = (A.fp - B.fp + mod) % mod; long nf0 = (A.f0 - B.f0 + mod) % mod; long nf1 = (A.f1 - B.f1 + mod) % mod; return new Datum(nf1, nf0, nfp); } public static Datum promote(Datum A) { return new Datum( (A.f1 + 1) % mod, A.f0, (A.fp + 1) % mod ); } public static long invl(long a, long mod) { long b = mod; long p = 1, q = 0; while (b > 0) { long c = a / b; long d; d = a; a = b; b = d % b; d = p; p = q; q = d - c * q; } return p < 0 ? p + mod : p; } public static int[][] parents(int[][] g, int root) { int n = g.length; int[] par = new int[n]; Arrays.fill(par, -1); int[] depth = new int[n]; depth[0] = 0; int[] q = new int[n]; q[0] = root; for (int p = 0, r = 1; p < r; p++) { int cur = q[p]; for (int nex : g[cur]) { if (par[cur] != nex) { q[r++] = nex; par[nex] = cur; depth[nex] = depth[cur] + 1; } } } return new int[][]{par, q, depth}; } public static int[][] packU(int n, int[] from, int[] to) { return packU(n, from, to, from.length); } public static int[][] packU(int n, int[] from, int[] to, int sup) { int[][] g = new int[n][]; int[] p = new int[n]; for (int i = 0; i < sup; i++) p[from[i]]++; for (int i = 0; i < sup; i++) p[to[i]]++; for (int i = 0; i < n; i++) g[i] = new int[p[i]]; for (int i = 0; i < sup; i++) { g[from[i]][--p[from[i]]] = to[i]; g[to[i]][--p[to[i]]] = from[i]; } return g; } public static void main(String[] args) { new H2().run(); } public void run() { long S = System.currentTimeMillis(); is = INPUT.isEmpty() ? System.in : new ByteArrayInputStream(INPUT.getBytes()); out = new FastWriter(System.out); solve(); out.flush(); long G = System.currentTimeMillis(); tr(G-S+"ms"); // Thread t = new Thread(null, null, "~", Runtime.getRuntime().maxMemory()){ // @Override // public void run() { // long s = System.currentTimeMillis(); // solve(); // out.flush(); // if(!INPUT.isEmpty())tr(System.currentTimeMillis()-s+"ms"); // } // }; // t.start(); // t.join(); } private boolean eof() { if(lenbuf == -1)return true; int lptr = ptrbuf; while(lptr < lenbuf)if(!isSpaceChar(inbuf[lptr++]))return false; try { is.mark(1000); while(true){ int b = is.read(); if(b == -1){ is.reset(); return true; }else if(!isSpaceChar(b)){ is.reset(); return false; } } } catch (IOException e) { return true; } } private final byte[] inbuf = new byte[1024]; public int lenbuf = 0, ptrbuf = 0; private int readByte() { if(lenbuf == -1)throw new InputMismatchException(); if(ptrbuf >= lenbuf){ ptrbuf = 0; try { lenbuf = is.read(inbuf); } catch (IOException e) { throw new InputMismatchException(); } if(lenbuf <= 0)return -1; } return inbuf[ptrbuf++]; } private boolean isSpaceChar(int c) { return !(c >= 33 && c <= 126); } // private boolean isSpaceChar(int c) { return !(c >= 32 && c <= 126); } private int skip() { int b; while((b = readByte()) != -1 && isSpaceChar(b)); return b; } private double nd() { return Double.parseDouble(ns()); } private char nc() { return (char)skip(); } private String ns() { int b = skip(); StringBuilder sb = new StringBuilder(); while(!(isSpaceChar(b))){ sb.appendCodePoint(b); b = readByte(); } return sb.toString(); } private char[] ns(int n) { char[] buf = new char[n]; int b = skip(), p = 0; while(p < n && !(isSpaceChar(b))){ buf[p++] = (char)b; b = readByte(); } return n == p ? buf : Arrays.copyOf(buf, p); } private char[][] nm(int n, int m) { char[][] map = new char[n][]; for(int i = 0;i < n;i++)map[i] = ns(m); return map; } private int[] na(int n) { int[] a = new int[n]; for(int i = 0;i < n;i++)a[i] = ni(); return a; } private long[] nal(int n) { long[] a = new long[n]; for(int i = 0;i < n;i++)a[i] = nl(); return a; } private int ni() { int num = 0, b; boolean minus = false; while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-')); if(b == '-'){ minus = true; b = readByte(); } while(true){ if(b >= '0' && b <= '9'){ num = num * 10 + (b - '0'); }else{ return minus ? -num : num; } b = readByte(); } } private long nl() { long num = 0; int b; boolean minus = false; while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-')); if(b == '-'){ minus = true; b = readByte(); } while(true){ if(b >= '0' && b <= '9'){ num = num * 10 + (b - '0'); }else{ return minus ? -num : num; } b = readByte(); } } public static class FastWriter { private static final int BUF_SIZE = 1<<13; private final byte[] buf = new byte[BUF_SIZE]; private final OutputStream out; private int ptr = 0; private FastWriter(){out = null;} public FastWriter(OutputStream os) { this.out = os; } public FastWriter(String path) { try { this.out = new FileOutputStream(path); } catch (FileNotFoundException e) { throw new RuntimeException("FastWriter"); } } public FastWriter write(byte b) { buf[ptr++] = b; if(ptr == BUF_SIZE)innerflush(); return this; } public FastWriter write(char c) { return write((byte)c); } public FastWriter write(char[] s) { for(char c : s){ buf[ptr++] = (byte)c; if(ptr == BUF_SIZE)innerflush(); } return this; } public FastWriter write(String s) { s.chars().forEach(c -> { buf[ptr++] = (byte)c; if(ptr == BUF_SIZE)innerflush(); }); return this; } private static int countDigits(int l) { if (l >= 1000000000) return 10; if (l >= 100000000) return 9; if (l >= 10000000) return 8; if (l >= 1000000) return 7; if (l >= 100000) return 6; if (l >= 10000) return 5; if (l >= 1000) return 4; if (l >= 100) return 3; if (l >= 10) return 2; return 1; } public FastWriter write(int x) { if(x == Integer.MIN_VALUE){ return write((long)x); } if(ptr + 12 >= BUF_SIZE)innerflush(); if(x < 0){ write((byte)'-'); x = -x; } int d = countDigits(x); for(int i = ptr + d - 1;i >= ptr;i--){ buf[i] = (byte)('0'+x%10); x /= 10; } ptr += d; return this; } private static int countDigits(long l) { if (l >= 1000000000000000000L) return 19; if (l >= 100000000000000000L) return 18; if (l >= 10000000000000000L) return 17; if (l >= 1000000000000000L) return 16; if (l >= 100000000000000L) return 15; if (l >= 10000000000000L) return 14; if (l >= 1000000000000L) return 13; if (l >= 100000000000L) return 12; if (l >= 10000000000L) return 11; if (l >= 1000000000L) return 10; if (l >= 100000000L) return 9; if (l >= 10000000L) return 8; if (l >= 1000000L) return 7; if (l >= 100000L) return 6; if (l >= 10000L) return 5; if (l >= 1000L) return 4; if (l >= 100L) return 3; if (l >= 10L) return 2; return 1; } public FastWriter write(long x) { if(x == Long.MIN_VALUE){ return write("" + x); } if(ptr + 21 >= BUF_SIZE)innerflush(); if(x < 0){ write((byte)'-'); x = -x; } int d = countDigits(x); for(int i = ptr + d - 1;i >= ptr;i--){ buf[i] = (byte)('0'+x%10); x /= 10; } ptr += d; return this; } public FastWriter write(double x, int precision) { if(x < 0){ write('-'); x = -x; } x += Math.pow(10, -precision)/2; // if(x < 0){ x = 0; } write((long)x).write("."); x -= (long)x; for(int i = 0;i < precision;i++){ x *= 10; write((char)('0'+(int)x)); x -= (int)x; } return this; } public FastWriter writeln(char c){ return write(c).writeln(); } public FastWriter writeln(int x){ return write(x).writeln(); } public FastWriter writeln(long x){ return write(x).writeln(); } public FastWriter writeln(double x, int precision){ return write(x, precision).writeln(); } public FastWriter write(int... xs) { boolean first = true; for(int x : xs) { if (!first) write(' '); first = false; write(x); } return this; } public FastWriter write(long... xs) { boolean first = true; for(long x : xs) { if (!first) write(' '); first = false; write(x); } return this; } public FastWriter write(IntUnaryOperator f, int... xs) { boolean first = true; for(int x : xs) { if (!first) write(' '); first = false; write(f.applyAsInt(x)); } return this; } public FastWriter write(LongUnaryOperator f, long... xs) { boolean first = true; for(long x : xs) { if (!first) write(' '); first = false; write(f.applyAsLong(x)); } return this; } public FastWriter writeln() { return write((byte)'\n'); } public FastWriter writeln(int... xs) { return write(xs).writeln(); } public FastWriter writeln(long... xs) { return write(xs).writeln(); } public FastWriter writeln(IntUnaryOperator f, int... xs) { return write(f, xs).writeln(); } public FastWriter writeln(LongUnaryOperator f, long... xs) { return write(f, xs).writeln(); } public FastWriter writeln(char[] line) { return write(line).writeln(); } public FastWriter writeln(char[]... map) { for(char[] line : map)write(line).writeln();return this; } public FastWriter writeln(String s) { return write(s).writeln(); } private void innerflush() { try { out.write(buf, 0, ptr); ptr = 0; } catch (IOException e) { throw new RuntimeException("innerflush"); } } public void flush() { innerflush(); try { out.flush(); } catch (IOException e) { throw new RuntimeException("flush"); } } public FastWriter print(byte b) { return write(b); } public FastWriter print(char c) { return write(c); } public FastWriter print(char[] s) { return write(s); } public FastWriter print(String s) { return write(s); } public FastWriter print(int x) { return write(x); } public FastWriter print(long x) { return write(x); } public FastWriter print(double x, int precision) { return write(x, precision); } public FastWriter println(char c){ return writeln(c); } public FastWriter println(int x){ return writeln(x); } public FastWriter println(long x){ return writeln(x); } public FastWriter println(double x, int precision){ return writeln(x, precision); } public FastWriter print(int... xs) { return write(xs); } public FastWriter print(long... xs) { return write(xs); } public FastWriter print(IntUnaryOperator f, int... xs) { return write(f, xs); } public FastWriter print(LongUnaryOperator f, long... xs) { return write(f, xs); } public FastWriter println(int... xs) { return writeln(xs); } public FastWriter println(long... xs) { return writeln(xs); } public FastWriter println(IntUnaryOperator f, int... xs) { return writeln(f, xs); } public FastWriter println(LongUnaryOperator f, long... xs) { return writeln(f, xs); } public FastWriter println(char[] line) { return writeln(line); } public FastWriter println(char[]... map) { return writeln(map); } public FastWriter println(String s) { return writeln(s); } public FastWriter println() { return writeln(); } } public static void trnz(int... o) { for(int i = 0;i < o.length;i++)if(o[i] != 0)System.out.print(i+":"+o[i]+" "); System.out.println(); } // print ids which are 1 public static void trt(long... o) { Queue stands = new ArrayDeque<>(); for(int i = 0;i < o.length;i++){ for(long x = o[i];x != 0;x &= x-1)stands.add(i<<6|Long.numberOfTrailingZeros(x)); } System.out.println(stands); } public static void tf(boolean... r) { for(boolean x : r)System.out.print(x?'#':'.'); System.out.println(); } public static void tf(boolean[]... b) { for(boolean[] r : b) { for(boolean x : r)System.out.print(x?'#':'.'); System.out.println(); } System.out.println(); } public void tf(long[]... b) { if(INPUT.length() != 0) { for (long[] r : b) { for (long x : r) { for (int i = 0; i < 64; i++) { System.out.print(x << ~i < 0 ? '#' : '.'); } } System.out.println(); } System.out.println(); } } public void tf(long... b) { if(INPUT.length() != 0) { for (long x : b) { for (int i = 0; i < 64; i++) { System.out.print(x << ~i < 0 ? '#' : '.'); } } System.out.println(); } } private void tr(Object... o) { if(INPUT.length() != 0)System.out.println(Arrays.deepToString(o)); } }