#include <bits/stdc++.h>
#define N 26
using namespace std;
double t[N], a[N][N];
int comb(int n, int k){
	int f[N];
	f[0]=1;
	for(int i=1;i<=n;i++) f[i]=f[i-1]*i;
	return(f[n]/(f[k]*f[n-k]));
}
int main()
{
    freopen("dezastru.in", "r", stdin);
    freopen("dezastru.out", "w", stdout);
    int i, n, k, j;
    scanf("%d%d", &n, &k);
    for(i=1;i<=n;i++) {
    	scanf("%lf", &t[i]);
	}
    a[1][1]=t[1];
    a[1][0]=1;
	for(i=2;i<=n;i++){
		a[i][0]=1;
		for(j=1;j<=k;j++){
			a[i][j]=a[i-1][j]+t[i]*a[i-1][j-1];
		}
	}
	printf("%lf ", a[n][k]/comb(n, k));
	return 0;
}