// dezstru.cpp : Defines the entry point for the console application.
//

#include <stdio.h>
int n,k,val[30],cont=0;
float l,e[30];
double inceput=1,sfarsit=1,t=0;
	
int main()
{
	int i,h,j;
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	scanf("%d %d",&n,&k);
	for(i=1;i<=n;i++)
		scanf("%f ",&e[i]);
	for(i=1;i<=k;i++)
		inceput=inceput*e[i];
	t=inceput;
	cont=1+k*(n-k);
	for(i=k;i>=1;i--)
	{
		inceput=inceput/e[i];
		for(j=i+1;j<=n-k+i;j++)
			t=t+inceput*e[j]*sfarsit;
		sfarsit=sfarsit*e[j-1];
	}
	l=t/cont;
	printf("%f",l);
	return 0;
}