#include <bits/stdc++.h>

using namespace std;

namespace {

    typedef double real;
    typedef long long ll;

    template<class T> ostream& operator<<(ostream& os, const vector<T>& vs) {
        if (vs.empty()) return os << "[]";
        os << "[" << vs[0];
        for (int i = 1; i < vs.size(); i++) os << " " << vs[i];
        return os << "]";
    }
    template<class T> istream& operator>>(istream& is, vector<T>& vs) {
        for (auto it = vs.begin(); it != vs.end(); it++) is >> *it;
        return is;
    }

    int to_i(string s) {
        istringstream is(s);
        int n; is >> n;
        return n;
    }

    int N, L;
    vector<int> S;
    void input() {
        cin >> N >> L; L *= 60;
        S.resize(N);
        for (int i = 0; i < N; i++) {
            string buf; cin >> buf;
            int m = to_i(buf.substr(0, 2));
            int s = to_i(buf.substr(3, 2));
            S[i] = m * 60 + s;
        }
    }

    ll COMB[100][100];
    void init(int N, int K) {
        for (int i = 0; i <= N; i++) for (int j = 0; j <= K; j++) COMB[i][j] = 0;
        COMB[0][0] = 1;
        for (int n = 1; n <= N; n++) {
            COMB[n][0] = 1;
            for (int k = 1; k <= K; k++) {
                COMB[n][k] = (COMB[n - 1][k - 1] + COMB[n - 1][k]);
            }
        }
    }

    ll comb(int N, int K) {
        if (N < K) return 0;
        return COMB[N][K];
    }

    int sum(int bit, const vector<int>::iterator& from, const vector<int>::iterator& to) {
        int ans = 0;
        int n = to - from;
        for (int i = 0; i < n; i++) {
            if (bit & (1 << i)) {
                ans += *(from + i);
            }
        }
        return ans;
    }

    void solve() {

        init(N, N);

        vector< vector<ll> > A(N + 1, vector<ll>(N + 1, 0));

        for (int i = 0; i < N; i++) {
            vector<int> ss;
            for (int j = 0; j < N; j++) {
                if (i == j) continue;
                ss.push_back(S[j]);
            }

            int M1 = (N - 1) / 2;
            int M2 = (N - 1) - M1;

            vector< vector<int> > xs(N);
            for (int bit = 0; bit < (1 << M2); bit++) {
                int s = sum(bit, ss.begin() + M1, ss.end());
                //cout << bit << " && " << s << endl;
                xs[__builtin_popcount(bit)].push_back(s);
            }
            for (int j = 0; j < N; j++) {
                sort(xs[j].begin(), xs[j].end());
                //cout << j << " -> " << xs[j] << endl;
            }

            for (int bit = 0; bit < (1 << M1); bit++) {
                int s = sum(bit, ss.begin(), ss.begin() + M1);
                int cnt = __builtin_popcount(bit);
                for (int j = 0; j < N; j++) {
                    int y = upper_bound( xs[j].begin(), xs[j].end(),  L - s - 1 ) - xs[j].begin();
                    A[i][cnt + j] += y;
                }
            }
        }

        real ans = 0;
        for (int i = 0; i < N; i++) {
            for (int k = 0; k < N; k++) {
                ans += (1.0 / N) * (real(A[i][k]) / comb(N - 1, k));
            }
        }
        cout << setprecision(15) << ans << endl;
    }
}

int main() {
    input(); solve();
    return 0;
}