#include <cstdio>
using namespace std;
int n, m;
long double p[310];
long double dp[310][310], f[310];
int main() {
    scanf("%d%d", &n, &m);
    for (int i = 0; i < n; i++) {
        scanf("%Lf", &p[i]);
        // p[0] -= p[i];
    }
    dp[m][0] = 1;
    f[m] = p[0];
    for (int i = m - 1; i >= 0; i--) {
        for (int j = 0; j <= n; j++) {
            if (j == 0) dp[i][0] = 1;
            else dp[i][j] = dp[i][j - 1] * f[i + 1];
            f[i] += p[j] * dp[i][j];
        }
    }
    printf("%.4Lf\n", f[1]);
    return 0;
}