#include <stdio.h>

int n, k, i, j;
double rez, a[26][26], v[26];

int main () {
	freopen("dezastru.in", "r", stdin);
	freopen("dezastru.out", "w", stdout);
	scanf("%d %d", &n, &k);
	for (i=1; i<=n; i++) scanf("%lf", &v[i]);
	for (i=0; i<n; i++) a[i][0]=1;
	for (i=1; i<=n; i++)
		for (j=1; j<=k; j++)
			a[i][j]=a[i-1][j]+a[i-1][j-1]*v[i];
	rez=a[n][k];
	for (i=k+1; i<=n; i++) rez/=i;
    for (i=2; i<=n-k; i++) rez*=i;
	printf("%.6lf", rez);
	return 0;
}