import core.thread;
import std.conv, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.container, std.math, std.range, std.regex;

//	Input
class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { tokens = readln.split; if (tokens.empty && stdin.eof) throw new EOFException; } auto token = tokens[0]; tokens.popFront; return token; }
int readInt() { return to!int(readToken); }
long readLong() { return to!long(readToken); }
real readReal() { return to!real(readToken); }

//	chmin/chmax
void chmin(T)(ref T t, in T f) { if (t > f) t = f; }
void chmax(T)(ref T t, in T f) { if (t < f) t = f; }

//	Pair
struct Pair(S, T) {
	S x; T y;
	int opCmp(    const Pair p) const { return (x < p.x) ? -1 : (x > p.x) ? +1 : (y < p.y) ? -1 : (y > p.y) ? +1 : 0; }
	int opCmp(ref const Pair p) const { return (x < p.x) ? -1 : (x > p.x) ? +1 : (y < p.y) ? -1 : (y > p.y) ? +1 : 0; }
	string toString() const { return "(" ~ to!string(x) ~ ", " ~ to!string(y) ~ ")"; }
}
auto pair(S, T)(inout(S) x, inout(T) y) { return Pair!(S, T)(x, y); }

//	Array
int binarySearch(T)(in T[] as, in bool delegate(T) test) { int low = -1, upp = as.length; for (; low + 1 < upp; ) { int mid = (low + upp) >> 1; (test(as[mid]) ? low : upp) = mid; } return upp; }
int lowerBound(T)(in T[] as, in T val) { return as.binarySearch((T a) { return (a <  val); }); }
int upperBound(T)(in T[] as, in T val) { return as.binarySearch((T a) { return (a <= val); }); }
T[] unique(T)(in T[] as) { T[] bs; foreach (a; as) if (bs.empty || bs[$ - 1] != a) bs ~= a; return bs; }


int N, L;
int[] A;

void main(string[] args) {
	try {
	for (; ; ) {
		N = readInt;
		L = readInt * 60;
		A = new int[N];
		foreach (i; 0 .. N) {
			auto ss = readToken.split(":");
			A[i] = ss[0].to!int * 60 + ss[1].to!int;
		}
		
		/*
		int[] perm = new int[N];
		foreach (i; 0 .. N) {
			perm[i] = i;
		}
		real nmr = 0.0;
		real dnm = 0.0;
		do {
			int s;
			foreach (j; 0 .. N) {
				nmr += 1.0;
				s += A[perm[j]];
				if (s >= L) {
					break;
				}
			}
			dnm += 1.0;
		} while (perm.nextPermutation);
		writeln(nmr / dnm);
		*/
		
		real ans = 0.0;
		foreach (i; 0 .. N) {
			auto crt = new real[][](1, L);
			crt[0][] = 0.0;
			crt[0][0] = 1.0;
			int l = 1;
			foreach (j; 0 .. N) if (j != i) {
				auto nxt = new real[][](l + 1, L);
				foreach (k; 0 .. l + 1) {
					nxt[k][] = 0.0;
				}
				foreach (k; 0 .. l) foreach (x; 0 .. L) {
					const prob = 1.0 * (k + 1) / (l + 1);
					nxt[k][x] += (1.0 - prob) * crt[k][x];
					if (x + A[j] < L) {
						nxt[k + 1][x + A[j]] += prob * crt[k][x];
					}
				}
				++l;
				crt = nxt;
			}
			foreach (k; 0 .. l) foreach (x; 0 .. L) {
				ans += crt[k][x];
			}
		}
		writefln("%.10f", ans);
		
	}
	} catch (EOFException) {}
}