package contest210618; import java.io.*; import java.util.*; import java.util.function.IntUnaryOperator; import java.util.function.LongUnaryOperator; public class D2 { InputStream is; FastWriter out; // String INPUT = "7 5 1 2 2 3 4 5 5 6 1 2"; // String INPUT = "4 3 1 4 2 3 3 4"; String INPUT = ""; public void solve() { int n = ni(), m = ni(); int[] from = new int[m]; int[] to = new int[m]; for(int i = 0;i < m;i++){ from[i] = ni()-1; to[i] = ni()-1; } int[][] g = packD(n, from, to); DJSet ds = new DJSet(n); for(int i = 0;i < m;i++){ ds.union(from[i], to[i]); } int[] clus = decomposeToSCCNoRec(g); int[] cf = new int[n]; for(int i = 0;i < n;i++){ cf[clus[i]]++; } boolean[] ring = new boolean[n]; for(int i = 0;i < n;i++){ if(cf[clus[i]] >= 2){ ring[ds.root(i)] = true; } } int[] can = new int[n]; Arrays.fill(can, -1); List list = new ArrayList<>(); for(int i = 0;i < n;i++){ can[clus[i]] = i; } int[] prev = new int[n]; Arrays.fill(prev, -1); for(int i = 0;i < n;i++){ if(can[i] != -1 && !ring[ds.root(can[i])]){ if(prev[ds.root(can[i])] != -1){ list.add((can[i]+1) + " " + (can[prev[ds.root(can[i])]]+1)); } prev[ds.root(can[i])] = i; } } int[] pv = new int[n]; int[] first = new int[n]; Arrays.fill(pv, -1); Arrays.fill(first, -1); for(int i = 0;i < n;i++){ int root = ds.root(i); if(ring[root]){ if(first[root] == -1)first[root] = i; if(pv[root] != -1){ list.add((pv[root]+1) + " " + (i+1)); } pv[root] = i; } } for(int i = 0;i < n;i++){ if(first[i] != -1) { list.add((pv[i] + 1) + " " + (first[i] + 1)); } } out.println(list.size()); for(String u : list){ out.println(u); } // check(list, g); } void check(List ans, int[][] g) { int m = ans.size(); int[] from = new int[m]; int[] to = new int[m]; int p = 0; for(String line : ans){ String[] sp = line.split(" "); from[p] = Integer.parseInt(sp[0])-1; to[p] = Integer.parseInt(sp[1])-1; p++; } int n = g.length; int[][] h = packD(n, from, to); for(int i = 0;i < n;i++) { for(int e : g[i]){ // i -> e assert (reachable(i, e, h)); } } } boolean reachable(int a, int b, int[][] g) { int n = g.length; boolean[] ved = new boolean[n]; Queue q = new ArrayDeque<>(); q.add(a); ved[a] = true; while(!q.isEmpty()){ int cur = q.poll(); for(int e : g[cur]){ if(!ved[e]){ ved[e] = true; q.add(e); } } } return ved[b]; } public static class DJSet { public int[] upper; public DJSet(int n) { upper = new int[n]; Arrays.fill(upper, -1); } public int root(int x) { return upper[x] < 0 ? x : (upper[x] = root(upper[x])); } public boolean equiv(int x, int y) { return root(x) == root(y); } public boolean union(int x, int y) { x = root(x); y = root(y); if (x != y) { if (upper[y] < upper[x]) { int d = x; x = y; y = d; } upper[x] += upper[y]; upper[y] = x; } return x == y; } public int count() { int ct = 0; for (int u : upper) if (u < 0) ct++; return ct; } public int[][] toBucket() { int n = upper.length; int[][] ret = new int[n][]; int[] rp = new int[n]; for (int i = 0; i < n; i++) if (upper[i] < 0) ret[i] = new int[-upper[i]]; for (int i = 0; i < n; i++) { int r = root(i); ret[r][rp[r]++] = i; } return ret; } } public static int[] decomposeToSCCNoRec(int[][] g) { int n = g.length; int[] stack = new int[n+1]; int[] ind = new int[n+1]; int[] ord = new int[n]; Arrays.fill(ord, -1); int[] low = new int[n]; Arrays.fill(low, -1); int sp = 0; int id = 0; // preorder int[] clus = new int[n]; int cid = 0; int[] cstack = new int[n+1]; int csp = 0; boolean[] incstack = new boolean[n]; for(int i = 0;i < n;i++){ if(ord[i] == -1){ ind[sp] = 0; cstack[csp++] = i; stack[sp++] = i; incstack[i] = true; while(sp > 0){ int cur = stack[sp-1]; if(ind[sp-1] == 0){ ord[cur] = low[cur] = id++; } if(ind[sp-1] < g[cur].length){ int nex = g[cur][ind[sp-1]]; if(ord[nex] == -1){ ind[sp-1]++; ind[sp] = 0; incstack[nex] = true; cstack[csp++] = nex; stack[sp++] = nex; }else{ // shortcut // U.tr(cur, nex, incstack[nex], low[nex], stack); if(incstack[nex])low[cur] = Math.min(low[cur], ord[nex]); // low is ok too, but slower. ind[sp-1]++; } }else{ if(ord[cur] == low[cur]){ while(csp > 0){ incstack[cstack[csp-1]] = false; clus[cstack[--csp]] = cid; if(cstack[csp] == cur)break; } cid++; } if(--sp >= 1)low[stack[sp-1]] = Math.min(low[stack[sp-1]], low[stack[sp]]); } } } } return clus; } public static int[][] packD(int n, int[] from, int[] to) { return packD(n, from, to, from.length); } public static int[][] packD(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 < n; i++) g[i] = new int[p[i]]; for (int i = 0; i < sup; i++) { g[from[i]][--p[from[i]]] = to[i]; } return g; } public static void main(String[] args) { new D2().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)); } }