#include <bits/stdc++.h>
using namespace std;

#define all(x) (x).begin(), (x).end()
using ll = long long;
using db = long double;

const int N = 105;
db f[105][1<<16], g[106][1<<16];
ll all, n, m, popcnt[1<<16];
ll sum[1<<16];
ll a[105];

db c[N][N];
void init()
{
    c[0][0] = 1;
    for (int i = 1; i < N; ++i)
    {
        c[i][0] = c[i][i] = 1;
        for (int j = 1; j < N; ++j) c[i][j] = c[i-1][j-1] + c[i-1][j];
    }
}
db C(int x, int y)
{
    if (x < 0 || y < 0 || x < y) return 0;
    return c[x][y];
}

int main()
{
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    cin >> n >> m; init();
    for (int i = 1; i <= n; ++i) cin >> a[i];
    all = (1<<n) - 1;
    for (int i = 1; i <= all; ++i) 
    {
        popcnt[i] = __popcount(i);
        sum[i] = sum[i - (i&-i)] + a[__lg(i&-i) + 1];
    }
    f[0][0] = 1;
    for (int i = 1; i <= m; ++i) f[i][0] = f[i-1][0] / n;
    g[0][0] = 1;
    for (int s = 1; s <= all; ++s)
    {
        int ss = s - (s&-s), id = __lg(s&-s) + 1;
        for (int i = 0; i < a[id]; ++i)
        for (int j = 0; j + i <= m; ++j)
            g[i+j][s] += g[j][ss] * C(i+j, i);
    }
    for (int s = 1; s <= all; ++s)
    {
        for (int i = 1; i <= n; ++i) for (int step = 1; step <= m; ++step) if (s>>(i-1) & 1)
        {
            f[step][s] += f[step-1][s - (1<<(i-1))] * C(step - sum[s - (1<<(i-1))] - 1, a[i] - 1) / (n - popcnt[s] + 1);
        }
        for (int i = 1; i <= m; ++i) f[i][s] += f[i-1][s] / (n - popcnt[s]);
    }
    db ans = 0;
    for (int s = 1; s <= all; ++s) if (m >= sum[s])
    {
        // cerr << s << " " << popcnt[s] << " " << f[m][s] << " " << g[m-sum[s]][all-s] << " "<< sum[s] << "\n";
        ans += popcnt[s] * f[m][s] * g[m - sum[s]][all - s];
    }
    
    cout << fixed << setprecision(16) << ans << "\n";
}