#include <cstdio>
#define min(a,b) (a < b ? a : b)
using namespace std;

const int nmx = 30;

int n,k;
double p[nmx],dp[nmx][nmx];

double combinari(int n1, int k1){
    double val = 1;
    for(int i = k1 + 1; i <= n1; ++i)
        val *= i;
    for(int i = 2; i <= n1 - k1; ++i)
        val /= i;
    return val;
}

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", &p[i]);

    for(int i = 1; i <= n; ++i){
        dp[i][1] = p[i] + dp[i-1][1];
        for(int j = 2; j <= min(i,k); ++j)
            dp[i][j] = dp[i-1][j] + dp[i-1][j-1] * p[i];
    }

    printf("%lf\n", dp[n][k] / combinari(n,k) );

    return 0;
}