#include<stdio.h>
int c[27][27],n,k,i,j;
double a[27][27],v[27];
int main ()
{
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	scanf("%d%d",&n,&k);
	for(i=0;i<n;++i)
		scanf("%lf",&v[i]);
	a[0][1]=1;
	c[0][1]=1;
	for(i=1;i<=n;++i)
		for(j=1;j<=k+1;++j){
			a[i][j]=a[i-1][j]+a[i-1][j-1]*v[i-1];
			c[i][j]=c[i-1][j-1]+c[i-1][j];
		}
	printf("%.6lf",a[n][k+1]/c[n][k+1]);
	return 0;
}