import std.algorithm, std.conv, std.range, std.stdio, std.string;
import std.bitmanip;  // BitArray

void main()
{
  auto rd = readln.split.to!(int[]), n = rd[0], l = rd[1] * 60;
  auto si = readln.split.map!(s => s[0..2].to!int * 60 + s[3..5].to!int).array;

  if (l >= si.sum) {
    writeln(n);
    return;
  }

  auto dp1 = new int[](n);
  
  auto calc(int t, int[] ti)
  {
    auto dp2 = BitArray();
    dp2.length = l + 1;
    dp2[0] = true;

    foreach (i; 0..n-1) {
      auto a = dp2.dup;
      a <<= ti[i];
      dp2 |= a;
    }

    auto rt = new int[][](l + 1);
    rt[0] = [0];
    foreach (i; 0..n-1)
      foreach_reverse (j; ti[i]..l+1)
        if (dp2[j - ti[i]] && dp2[j])
          rt[j] ~= rt[j - ti[i]].map!"a + 1".array;

    foreach (j; l-t..l)
      foreach (r; rt[j])
        ++dp1[r];
  }

  foreach (i; 0..n) {
    auto t = si[i], ti = si[0..i] ~ si[i+1..n];
    calc(t, ti);
  }

  real r = 0;
  foreach (i; 0..n) {
    auto a = dp1[i].to!real * (i + 1);
    foreach (j; 1..i+1) a *= j;
    foreach (j; 1..n-i) a *= j;
    foreach (j; 1..n+1) a /= j;
    r += a;
  }

  writefln("%.10f", r);
}