#include "bits/stdc++.h"
#define rep(i,n) for(int i=0;i<n;i++)
#define ALL(v) (v).begin(),(v).end()
typedef long long LL;
const LL INF = (1LL << 31) - 1;
const LL MOD = 1000000007LL;
using namespace std;
double dp[51][18001];
double dp2[51][18001];
double fact[51];
int main() {
	int t[50];
	int N, L;
	cin >> N >> L;
	L *= 60;
	fact[0] = 1;
	for (int i = 1; i <= N; i++) fact[i] = fact[i - 1] * i;
	int sum = 0;
	rep(i, N) {
		int mm, ss;
		scanf("%d:%d", &mm, &ss);
		t[i] = mm * 60 + ss;
		sum += t[i];
	}
	if (sum <= L) {
		cout << N << endl;
		return 0;
	}
	dp[0][0] = 1;
	rep(i, N) {
		for(int j=N-1;j>=0;j--) {
			for (int k = 0; k <= L; k++) {
				if (k - t[i] >= 0) dp[j + 1][k] += dp[j][k - t[i]];
			}
		}
	}
	double ret = 0;
	rep(i, N) {
		rep(j, N) {
			for (int k = 0; k <= L-1; k++) {
				dp2[j][k] = dp[j][k];
				if (j > 0 && k >= t[i]) dp2[j][k] -= dp2[j - 1][k - t[i]];
				ret += dp2[j][k] * fact[j] * fact[N - 1 - j];
			}
		}
	}
	printf("%.15lf\n", ret / fact[N]);
}