#include <fstream>
#include <iostream>
#include <iomanip>

using namespace std;

ifstream f("dezastru.in");
ofstream g("dezastru.out");

int n, k;
double dp[30][30], pascal[30][30], sum[30], v[30];

void precalc()
{   pascal[0][0] = 1;
    for(int i=1; i<=25; i++)
        {   pascal[i][0] = 1;
            for(int j=1; j<=25; j++) pascal[i][j] = pascal[i-1][j] + pascal[i-1][j-1];
        }
}

int main()
{
    g << setprecision(6) << fixed;
    precalc();
    f >> n >> k;
    for(int i=1; i<=n; i++) f >> v[i];
    sum[0] = 1;
    for(int i=1; i<=n; i++)
        {   for(int j=1; j<=min(i, k); j++) dp[i][j] = v[i] * sum[j-1];
            for(int j=1; j<=min(i, k); j++) sum[j] += dp[i][j];
        }
    g << sum[k] / pascal[n][k] << '\n';
    return 0;
}