#include <stdio.h>
#include <math.h>
#define Nmax 26

float a[Nmax][Nmax],v[Nmax],c[Nmax][Nmax];
int n,k,i,j;
int e[Nmax];

int main(){
	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]);

   for(i=0;i<=n;++i) a[i][0]=1,c[i][0]=1;
   for(i=1; i<=n; ++i)
     for(j=1; j<=i; ++j){
     	a[i][j] = a[i-1][j] + a[i-1][j-1] * v[i];
      c[i][j] = c[i-1][j] + c[i-1][j-1];
     }

   printf("%.6f\n",a[n][k]/c[n][k]);
   fclose(stdin); fclose(stdout);
   return 0;
}