#include <stdio.h>

using namespace std;

const int MAX = 30;

int n, k;
double v[MAX], dp[MAX][MAX], c[MAX][MAX];

void citire()
{
    freopen("dezastru.in", "r", stdin);
    scanf("%d %d", &n, &k);
    for(int i = 1; i <= n; i++)
        scanf("%lf", &v[i]);
    fclose(stdin);
}

void solve()
{
    for(int i = 1; i <= n; i++)
    {
        dp[i][1] = dp[i - 1][1] + v[i];
        c[i][1] = i;
        for(int j = 2; j <= k; j++)
        {
            dp[i][j] = dp[i - 1][j] + dp[i - 1][j - 1] * v[i];
            c[i][j] = c[i - 1][j - 1] + c[i - 1][j];
        }
    }
}

void afisare()
{
    freopen("dezastru.out", "w", stdout);
    printf("%.6lf", dp[n][k] / (double)c[n][k]);
    fclose(stdout);
}

int main()
{
    citire();
    solve();
    afisare();
    return 0;
}