package contest210507; import java.io.*; import java.math.BigDecimal; import java.math.BigInteger; import java.util.*; import java.util.function.IntUnaryOperator; import java.util.function.LongUnaryOperator; public class C { InputStream is; FastWriter out; // 1 1 1 1 1 2 1 4 String INPUT = ""; public void solve() { int n = ni(), K = ni(); int[] a = na(n); long[] nf = new long[101]; long[] f = new long[101]; for(int i = 0;i < n-1;i++){ f[a[i]*a[i+1]]++; if(i < n-K){ nf[a[i]*a[i+1]]++; } } FB den = new FB(0); for(int i = 1;i <= 100;i++){ den.add(new FB(f[i], i)); } FB num = new FB(0); for(int i = 1;i <= 100;i++){ num.add(new FB(nf[i], i)); } // tr(num, den); num = num.div(den); out.println(num.toString()); } public static class FB implements Comparable { public BigInteger num, den; public FB(){ this.num = BigInteger.ZERO; this.den = BigInteger.ONE; } public FB(long num) { this.num = BigInteger.valueOf(num); this.den = BigInteger.ONE; } public FB(long num, long den) { this.num = BigInteger.valueOf(num); this.den = BigInteger.valueOf(den); reduce(); } public FB(BigInteger num) { this.num = num; this.den = BigInteger.ONE;} public FB(BigInteger num, BigInteger den) { this.num = num; this.den = den; reduce();} public FB(BigDecimal d) { int scale = d.scale(); num = d.scaleByPowerOfTen(scale).toBigInteger(); den = BigInteger.valueOf(10).pow(scale); reduce(); } public FB neg(){ return new FB(num.negate(), den);} public FB abs(){ return new FB(num.abs(), den);} public FB reduce() { if(den.signum() == 0) { }else if(num.signum() == 0) { den = BigInteger.ONE; }else { if(den.signum() < 0) { num = num.negate(); den = den.negate(); } BigInteger g = num.gcd(den); num = num.divide(g); den = den.divide(g); } return this; } public static FB add(FB a, FB b){ return new FB(a.num.multiply(b.den).add(a.den.multiply(b.num)), a.den.multiply(b.den)); } public static FB sub(FB a, FB b){ return new FB(a.num.multiply(b.den).subtract(a.den.multiply(b.num)), a.den.multiply(b.den)); } public static FB mul(FB a, FB b){ return new FB(a.num.multiply(b.num), a.den.multiply(b.den)); } public static FB div(FB a, FB b){ return new FB(a.num.multiply(b.den), a.den.multiply(b.num)); } public static FB inv(FB a) { return new FB(a.den, a.num); } public FB add(FB b){ num = num.multiply(b.den).add(den.multiply(b.num)); den = den.multiply(b.den); return reduce(); } public FB sub(FB b){ num = num.multiply(b.den).subtract(den.multiply(b.num)); den = den.multiply(b.den); return reduce(); } public FB mul(FB b){ num = num.multiply(b.num); den = den.multiply(b.den); return reduce(); } public FB div(FB b){ num = num.multiply(b.den); den = den.multiply(b.num); return reduce(); } public FB inv() { BigInteger d = num; num = den; den = d; return reduce(); } public String toString() { return num.toString() + "/" + den.toString(); } public String toStringSimple() { return den.equals(BigInteger.ONE) ? num.toString() : num.toString() + "/" + den.toString(); } public int compareTo(FB f) { return num.multiply(f.den).compareTo(f.num.multiply(den)); } public boolean simpleEquals(FB f) { return num.equals(f.num) && den.equals(f.den); } public boolean equals(Object o) { if(o == null)return false; FB f = (FB) o; return num.multiply(f.den).equals(f.num.multiply(den)); } /** * 10進循環小数表記 * @param a * @return * @verified SRM761div1easy */ public static String circ(FB a) { BigInteger[] qr = a.num.divideAndRemainder(a.den); Map modstep = new HashMap<>(); BigInteger cur = qr[1]; int tail = -1; String P = ""; String t = ""; for(int i = 0;;i++){ if(cur.signum() == 0){ tail = i; P = "0"; break; } if(modstep.containsKey(cur)){ tail = modstep.get(cur); P = t.substring(modstep.get(cur)); break; } modstep.put(cur, i); BigInteger[] nqr = cur.multiply(BigInteger.TEN).divideAndRemainder(a.den); t += nqr[0].toString(); cur = nqr[1]; } return qr[0].toString() + "." + t.substring(0, tail) + "(" + P + ")"; } } public static long[] guessFrac(long n, int mod) { long min = mod; long argnum = -1, argden = 0; for(int den = 1;den <= 200000;den++){ long num = n*den%mod; if(num*2 >= mod)num -= mod; if(Math.abs(num) + den < min){ min = Math.abs(num) + den; argnum = num; argden = den; } } return argden == 0 ? null : new long[]{argnum, argden}; } 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 void main(String[] args) { new C().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)); } }