#include <bits/stdc++.h>

using namespace std;
typedef long double ld;

int n, k;
ld pd[30][30], pascal[30][30], v[30];

int main()
{
    ifstream fin ("dezastru.in");
    ofstream fout ("dezastru.out");
    pascal[0][0] = 1;
    for (int i = 1; i < 30; ++i){
        pascal[i][0] = 1;
        for (int j = 1; j <= i; ++j)
            pascal[i][j] = pascal[i-1][j] + pascal[i-1][j-1];
    }
    fin >> n >> k;
    for (int i = 1; i <= n; ++i)
        fin >> v[i];
    for (int i = 0; i <= n; ++i)
        pd[i][0] = 1;
    for (int i = 1; i <= n; ++i)
        for (int j = 1; j <= k; ++j)
            pd[i][j] = pd[i-1][j] + v[i]*pd[i-1][j-1];
    fout << fixed << setprecision(10) << pd[n][k]/pascal[n][k] << "\n";
    return 0;
}