#include<stdio.h>

float v[26];
int n,k;
float A[26][26];
float comb;

int main()
{
   freopen("dezastru.in","r",stdin);
   freopen("dezastru.out","w",stdout);

   scanf("%d%d",&n,&k);
   for(int i=1;i<=n;i++)
	   scanf("%f",&v[i]);
   for(int i=0;i<=25;i++)
	   A[i][0]=1;
   for(int i=1;i<=25;i++)
	   for(int j=1;j<=25;j++)
		   A[i][j]=A[i-1][j]+(A[i-1][j-1]*v[i]);
   comb=1;int aux=n-k;
   for(int i=k+1;i<=n;i++)
   {
	   comb*=(float)i/(float)aux;
	   if(aux>1)
		   aux--;
   }
   printf("%f",(float)A[n][k]/comb);
}