#include <stdio.h>

double a[26],p;
int nr,n,kk;

void bk(int ant, int k, double s){
     
     if (k==kk+1){p+=s;++nr;}
     else for (int i=ant+1;i<n-(kk-k)+1;i++) bk(i,k+1,s*a[i]);
}
     


int main()
{int i;
 freopen ("dezastru.in","r",stdin);
 freopen ("dezastru.out", "w",stdout);
 scanf("%d%d",&n,&kk);
 for (i=1;i<=n;i++) scanf("%lf", &a+i);
 bk(0,1,1);
 p/=nr;
 printf("%lf",p);
 return 0;
}