#include <fstream>
#include <iomanip>
#include <algorithm>

using namespace std;

int n, k;
int c[30][30];
double ar[30];
double dp[30][30];

int main()
{
    ifstream fi("dezastru.in");
    ofstream fo("dezastru.out");

    fi >> n >> k;
    for (int i = 1; i <= n; i++)
        fi >> ar[i];

    for (int i = 1; i <= n; i++)
        dp[i][1] = ar[i],
        c[i][1] = 1;

    for (int i = 2; i <= n; i++)
        for (int j = 1; j <= k; j++)
            dp[i][j] += dp[i - 1][j] + ar[i] * dp[i - 1][j - 1],
            c[i][j] += c[i - 1][j] + c[i - 1][j - 1];

    fo << setprecision(12) << (dp[n][k] / (double)c[n][k]);

  return 0;
}