#include <bits/stdc++.h>
using namespace std;
void solve() {
    int n, m;
    cin >> n >> m;
    double ans = 0;
    vector<int> a(n);
    vector<vector<double>> dp(1 << n, vector<double>(200, 0)), coef(1 << n, vector<double>(200, 0));
    vector<vector<double>> c(200, vector<double>(200, 0));
    for (int i = 0; i < n; ++i) cin >> a[i];
    vector<int> sum(1 << n);
    for (int i = 0; i <= 199; ++i) {
        c[i][0] = 1;
        for (int j = 1; j <= i; ++j) {
            c[i][j] = c[i - 1][j] + c[i - 1][j - 1];
        }
    }
    dp[0][0] = 1;
    for (int i = 1; i < m; i++)
        dp[0][i] = dp[0][i - 1] / n;
    coef[0][0] = 1;
    for (int sta = 1; sta < (1 << n); sta++) {
        int low = sta & (-sta), p = __builtin_ctz(sta);
        int cnt = n - __builtin_popcount(sta);
        sum[sta] = sum[sta ^ low] + a[p];
        for (int i = 0; i < n; i++)
            if ((sta >> i) & 1)
                for (int j = sum[sta]; j <= m; j++)
                    dp[sta][j] += dp[sta ^ (1 << i)][j - 1] / (cnt + 1) * c[j - sum[sta ^ (1 << i)] - 1][a[i] - 1];
        for (int i = sum[sta] + 1; i <= m; i++)
            dp[sta][i] += dp[sta][i - 1] / cnt;
        for (int i = 0; i < a[p]; i++)
            for (int j = 0; i + j <= m; j++)
                coef[sta][i + j] += c[i + j][j] * coef[sta ^ low][j];
    }
    for (int sta = 1; sta < (1 << n); sta++)
        if (sum[sta] <= m)
            ans += __builtin_popcount(sta) * dp[sta][m] * coef[((1 << n) - 1) ^ sta][m - sum[sta]];
    cout << fixed << setprecision(12) << ans << endl;
}
int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int t = 1;
    // cin >> t;
    while (t--) {
        solve();
    }
}