#include <stdio.h>

#define nmax 30
#define kmax 30

double p[nmax];
double a[nmax][kmax],c[nmax][kmax];
int n,k,i,j;

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",&p[i]);

	for(i = 0; i <= n; i++) a[i][0] = 1;

	for(j = 1; j <= k; j++) 
		for(i = 1; i <= n; i++) a[i][j] = a[i - 1][j] + a[i - 1][j - 1] * p[i];

	for(i = 1; i <= n; i++) {
		c[i][1] = i;
		for(j = 2; j <= k; j++) c[i][j] = c[i - 1][j] + c[i - 1][j - 1];
	}

	printf("%.6lf\n",a[n][k] / c[n][k]);

	return 0;
}