#define _USE_MATH_DEFINES
#include <algorithm>
#include <cstdio>
#include <functional>
#include <iostream>
#include <cfloat>
#include <climits>
#include <cstring>
#include <cmath>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <time.h>
#include <vector>
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> i_i;
typedef pair<ll, int> ll_i;
typedef pair<double, int> d_i;
typedef pair<ll, ll> ll_ll;
typedef pair<double, double> d_d;
struct edge { int v, w; };

ll MOD = 1000000007;
ll _MOD = 1000000009;
double EPS = 1e-10;
int INF = INT_MAX / 4;

int main() {
	int N, L;
	cin >> N >> L;
	L *= 60;
	vector<int> a(N);
	for (int i = 0; i < N; i++) {
		int MM, SS;
		scanf("%d:%d", &MM, &SS);
		a[i] = MM * 60 + SS;
	}
	double e = 0;
	for (int i = 0; i < N; i++) {
		vector<vector<double> > dp(N, vector<double>(L));
		dp[0][0] = 1;
		int z = 0;
		for (int j = 0; j < N; j++) {
			if (j == i) continue;
			vector<vector<double> > _dp(N, vector<double>(L));
			for (int l = 0; l <= z; l++)
				for (int k = 0; k < L; k++) {
					int x = l, y = z - l;
					_dp[l][k] += dp[l][k] * (y + 1) / (x + y + 2);
					if (k + a[j] < L) _dp[l + 1][k + a[j]] += dp[l][k] * (x + 1) / (x + y + 2);
				}
			dp = _dp;
			z++;
		}
		double sum = 0;
		for (int l = 0; l < N; l++)
			for (int k = 0; k < L; k++)
				sum += dp[l][k];
		e += sum;
	}
	printf("%.10f\n", e);
}