#include <cstdio>
using namespace std;
double c[26][26], s[26][26], v[26], ans;
int main()
{
    freopen("dezastru.in", "r", stdin);
    freopen("dezastru.out", "w", stdout);
    int n, k;
    scanf("%d %d", &n, &k);
    for (int i = 1; i <= n; ++i)
        scanf("%lf", &v[i]);
    for (int i = 0; i <= n; ++i)
        s[i][0] = c[i][0] = 1;
    for (int i = 1; i <= n; ++i)
        for (int j = 1; j <= k; ++j)
        {
            s[i][j] = s[i - 1][j] + s[i - 1][j - 1] * v[i];
            c[i][j] = c[i - 1][j] + c[i - 1][j - 1];
        }
    ans = s[n][k] / c[n][k];
    printf("%.6lf", ans);
    return 0;
}