#include <map>

#include <set>

#include <stack>

#include <cmath>

#include <queue>

#include <cstdio>

#include <vector>

#include <string>

#include <cstring>

#include <iterator>

#include <iostream>

#include <algorithm>

#define debug(a) cout << #a << " " << a << endl

using namespace std;

const int maxn = 2*1e2 + 10;

const int mod = 10000;

typedef long long ll;

ll f( ll x ) {

    ll sum = 1;

    for( ll i = 1; i <= x; i ++ ) {

        sum *= i;

    }

    return sum;

}

int main() {

    ios::sync_with_stdio(0),cin.tie(0),cout.tie(0);

    ll n, m, num[maxn];

    double a[maxn], b[maxn];

    while( cin >> n >> m ) {

        memset( a, 0, sizeof(a) );

        memset( b, 0, sizeof(b) );

        for( ll i = 0; i < n; i ++ ) {

            cin >> num[i];

        }

        for( ll i = 0; i <= num[0]; i ++ ) {

            a[i] = 1.0/f(i);

        }

        for( ll i = 1; i < n; i ++ ) {

            for( ll j = 0; j <= n; j ++ ) {

                for( ll k = 0; k <= num[i] && k+j <= n; k ++ ) {

                    b[k+j] += a[j]/f(k);

                }

            }

            for( ll j = 0; j <= n; j ++ ) {

                a[j] = b[j], b[j] = 0;

            }

        }

        printf("%.lf\n",a[m]*f(m) );

    }

    return 0;

}