#include <fstream>
#include <cstdio>
using namespace std;

const int N = 1001;
double a[N][N], p[N];
int n, k;

ifstream in("dezastru.in");

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

    for (int i = 1 ; i <= n ; i++)
        in >> p[i];

    for (int i = 0 ; i <= n ; i++)
        a[i][0] = 1;

    for (int i = 1 ; i <= n ; i++)
        for (int j = 1 ; j <= i ; j++)
            a[i][j] = (a[i - 1][j] * (i - j) + a[i - 1][j - 1] * p[i] * j) / i;

    freopen("dezastru.out", "w", stdout);

    printf("%.6lf\n", a[n][k]);

    return 0;
}