#include <iostream>
#include <cstdio>
#include<list>
#include<iomanip>
#include<cmath>
#include <random>
#include<queue>
#include <functional>
#include<stdio.h>
#include<assert.h>
#include<stack>
#include<sstream>
#include <cstdlib>
#include<map>
#include<algorithm>
#include<iostream>
#include<set>
#include<utility>
#include<memory.h>
#include<string>
#include<vector>
#include <unordered_map>
#include<numeric>
using namespace std;
#define ios std::ios_base::sync_with_stdio(false);
#define ll  long long
#define pb push_back
#define fi(ss) freopen (ss,"r",stdin)
#define fo(ss) freopen (ss,"w",stdout)
#define sz(v)               ((int)((v).size()))
#define all(x)          (x).begin(),(x).end()
#define REP(i, v)       for(int i=0;i<sz(v);i++)
#define lp(i,n) for(int i = 0 ; i < n ; ++i)
#define hash ___hash
#define next ___next
#define prev ___prev
#define left ___left
typedef pair<ll,ll> pii;
typedef pair<pii,int> tri;
int n,l;
const int maxSum = 25005;
int s[60];
double fac[60];
double dp1[maxSum + 1][60];
double dp2[maxSum + 1][60];

// set back dp
int main() {
	fac[0] = 1;
	for (int i = 1 ; i < 60 ; ++i) {
		fac[i] = 1.0*i * fac[i - 1];
	}
	scanf("%d %d",&n,&l);
	ll sum = 0;
	l*=60;
	int x,y;
	for (int i = 0 ; i < n ; ++i) {
		scanf("%d:%d",&x,&y);
		s[i]=x*60+y;
		sum+=s[i];
	}
	if (l >= sum) {
		return printf("%d\n",n);
	}

	dp1[0][0] = 1;
	for (int i = 0 ; i < n ; ++i) {
		for (int j = i; j >= 0 ; --j) {
			for (int cur = 0; cur <= maxSum ; ++cur) {

				dp1[min(maxSum,cur + s[i])][j + 1] += 1.0*dp1[cur][j];
			}
		}
	}
	double ways = 0.0;
	for (int i = 0 ; i < n ; ++i) {
		for (int j = 0 ; j <= n ; ++j) {
			for (int cur = 0 ; cur < l ;++cur) {
				dp2[cur][j] = dp1[cur][j];
				if (cur - s[i] >= 0 && j >= 1) {
					dp2[cur][j] -= dp2[cur - s[i]][j - 1];
				}
				if (j < n && cur + s[i] >= l) {
					ways += 1.0*(j+1)*(((dp2[cur][j] * 1.0*fac[j] * fac[n - j - 1])));
				}

			}
		}
	}

	printf("%.15lf\n",ways/fac[n]);


}