#include <fstream>
#include <iomanip>

using namespace std;

ifstream fin("dezastru.in");
ofstream fout("dezastru.out");

long double v[30], pr = 1, sf, aux, comb[30][30], dp[30][30], sum[30];
int lg, n, k;

int main()
{
    fin >> n >> k;

    for(int i = 1; i <= n; ++i)
        fin >> v[i];

    comb[0][0] = 1;
    for(int i = 1; i <= n; ++i){
        comb[i][0] = 1;
        comb[i][n] = 1;
        for(int j = 1; j < n; ++j)
            comb[i][j] = comb[i - 1][j - 1] + comb[i - 1][j];
    }

    dp[0][0] = 1;
    for(int i = 1; i <= n; ++i){
        dp[i][0] = 1;
        for(int j = 1; j <= i; ++j)
            dp[i][j] = (dp[i - 1][j] + dp[i - 1][j - 1] * v[i]);
    }

    fout << dp[n][k] / comb[n][k] << '\n';
    return 0;
}