# include <bits/stdc++.h>

using namespace std;

const int Nmax = 25 + 5;

int n, m, i, j;
int C[Nmax][Nmax];
double D[Nmax][Nmax], a[Nmax];

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

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

    for (i = 1; i <= n; ++i)
        scanf("%lf ", &a[i]);

    //permutari sume
    D[0][0] = 1;
    for (i = 1; i <= n; ++i)
    {
        D[i][0] = 1;
        for (j = 1; j <= i; ++j)
            D[i][j] = D[i-1][j] + a[i] * D[i-1][j-1];
    }

    //nr combinari

    C[0][0] = 1;
    for (i = 1; i <= n; ++i)
    {
        C[i][0] = 1;
        for (j = 1; j <= m; ++j)
            C[i][j] = C[i-1][j] + C[i-1][j-1];
    }

    double ans = (D[n][m] / C[n][m]);

    printf("%.6lf", ans);

    return 0;
}