#include <stdio.h>
int a[30][30],n,k;
float v[26],c[30][30];
void dinamica(){
	int i,j;
	a[0][1]=1;
	c[0][1]=1;
	k++;
	for(i=1;i<=n;i++)
		for(j=1;j<=k;j++){
			a[i][j]=a[i-1][j]+a[i-1][j-1];
			c[i][j]=c[i-1][j]+c[i-1][j-1]*v[i];
		}
	printf("%.6f",c[n][k]/a[n][k]);
}
int main(){
	int i;
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	scanf("%d%d",&n,&k);
	for(i=1;i<=n;i++)
		scanf("%f",&v[i]);
	dinamica();
	return 0;
}