#include <cstdio>
#include <cstring>

int n, k;
double v[25];

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

    scanf("%d %d", &n, &k);
    for (int i = 1; i <= n; ++i)
        scanf("%lf", &v[i]);
    double a[26][26];
    memset(a, 0, sizeof(a));
    a[0][0] = 1;
    for (int i = 0; i < n; ++i)
        for (int j = 0; j <= i; ++j) {
            a[i + 1][j] += a[i][j];
            a[i + 1][j + 1] += a[i][j] * v[i + 1];
        }
    int comb[26][26];
    comb[0][0] = 1;
    for (int i = 1; i <= n; ++i) {
        comb[i][0] = 1;
        for (int j = 1; j <= k; ++j)
            comb[i][j] = comb[i - 1][j] + comb[i - 1][j - 1];
    }
    printf("%lf\n", a[n][k] / comb[n][k]);

    return 0;
}