#include <cstdio>
using namespace std;
float s,q,w,a[100];
long n,k,i;
double suma(int c,double prod,int r)
{
	double t=0;
	long j;
	if(r==0){t=prod;}
	else{for(j=c;j<=n-r;j++){t+=suma(j+1,prod*a[j],r-1);}}
return t;
}
int main()
{
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	scanf("%d%d\n",&n,&k);
	for(i=1;i<=n;i++){
		scanf("%f",&a[i]);
	                 }
	s=suma(1,1,k);q=1;
	for(i=k;i<=n;i++){q*=i;}
	w=1/q;w*=s;printf("%f",w);
return 0;
}