#include <stdio.h>

double rez,p[30];
int nr,n,m;

void urm(int k, int i, float atac){

     if(k==m+1){
       rez+=atac;
       ++nr;
     }else
       for(;i<=n-m+k;++i)
	  urm(k+1,i+1,atac*p[i]);
}

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

    scanf("%d%d",&n,&m);

    for(i=1;i<=n;i++)
       scanf("%lf",&p[i]);

    urm(1,1,1);

    printf("%.6lf",rez/nr);

    return 0;
}