/**
 *    author:  otera    
**/
#include<iostream>
#include<string> 
#include<cstdio>
#include<cstring>
#include<vector>
#include<cmath>
#include<algorithm> 
#include<functional>
#include<iomanip>
#include<queue>
#include<deque>
#include<ciso646>
#include<random>
#include<map>
#include<set>
#include<complex>
#include<bitset>
#include<stack>
#include<unordered_map>
#include<unordered_set>
#include<utility>
#include<cassert>
#include<array>
using namespace std;

#define int long long
typedef long long ll;
typedef unsigned long long ul;
typedef unsigned int ui;
typedef long double ld;
const int inf=1e9+7;
const ll INF=1LL<<60 ;
const ll mod=1e9+7 ;
#define rep(i,n) for(int i=0;i<n;i++)
#define per(i,n) for(int i=n-1;i>=0;i--)
#define Rep(i,sta,n) for(int i=sta;i<n;i++)
#define rep1(i,n) for(int i=1;i<=n;i++)
#define per1(i,n) for(int i=n;i>=1;i--)
#define Rep1(i,sta,n) for(int i=sta;i<=n;i++)
typedef complex<ld> Point;
const ld eps = 1e-8;
const ld pi = acos(-1.0);
typedef pair<int, int> P;
typedef pair<ld, ld> LDP;
typedef pair<ll, ll> LP;
#define fr first
#define sc second
#define all(c) c.begin(),c.end()
#define pb push_back
#define debug(x)  cerr << #x << " = " << (x) << endl;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

double com(int n, int r) {
    double res = 1;
    rep(i, r) {
        res *= (n - i);
        res /= (i + 1);
    }
    return res;
}

void solve() {
	int n, l; cin >> n >> l;
    l *= 60;
    vector<int> s(n);
    rep(i, n) {
        string str; cin >> str;
        int mm = (str[0] - '0') * 10 + (str[1] - '0');
        int ss = (str[3] - '0') * 10 + (str[4] - '0');
        s[i] = mm * 60 + ss;
    }
    double res = 0.0;
    vector<vector<ll>> dp(n + 1, vector<ll>(l + 1, 0));
    dp[0][0] = 1;
    rep(i, n) {
        auto ndp = dp;
        for(int j = 1; j <= n; ++ j) {
            for(int k = s[i]; k <= l; ++ k) {
                ndp[j][k] += dp[j - 1][k - s[i]];
            }
        }
        swap(dp, ndp);
    }
    rep(i, n) {
        double tmp = 0.0;
        auto sdp = dp;
        for(int j = 1; j <= n; ++ j) {
            for(int k = s[i]; k <= l; ++ k) {
                sdp[j][k] -= sdp[j - 1][k - s[i]];
            }
        }
        for(int k = 0; k <= n - 1; ++ k) {
            double num = 0;
            for(int j = 0; j <= l - 1; ++ j) {
                num += sdp[k][j];
            }
            tmp += num / com(n - 1, k);
        }
        res += tmp / n; 
    }
    cout << res << endl;
}

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout << fixed << setprecision(10);
	//int t; cin >> t; rep(i, t)solve();
	solve();
    return 0;
}