#include <cstdio>
using namespace std;

const int Nmax = 25+2;

int main(){
    freopen("dezastru.in", "r", stdin);
    freopen("dezastru.out", "w", stdout);

    int n, m;
    scanf("%d%d", &n, &m);

    int i, j, l = 0;
    float dez, v[2][Nmax];
    v[0][0] = v[1][0] = 1;
    for (i = 1; i <= n; ++i) {
        scanf("%f", &dez);
        for (j = 1, l = 1-l; j <= i && j <= m; ++j) {
            v[l][j] = j * v[1-l][j-1] * dez;
            v[l][j]+= (i-j) * v[1-l][j];
            v[l][j]/= i;
        }
    }

    printf("%.*f", 6, v[l][m]);

    return 0;
}