#include <stdio.h>

char n,a[30],k,i;
float p,b[30];
int c=1;

void comb (int x,int m,double p1) {
    int i;
    if (x==k) {
        p+=p1;
        return;
    }
    for (i=m; i<=n-k+x+1; i++) comb (x+1,i+1,p1*b[i-1]);
}

int main () {
    freopen ("dezastru.in","r",stdin);
    freopen ("dezastru.out","w",stdout);
    scanf ("%d%d",&n,&k);
    for (i=0; i<n; i++) scanf ("%f",b+i);
    for (i=n-k+1; i<=n; i++) c*=i;
    //printf ("%d\n",c);
    comb (0,1,1);
    printf ("%.6f\n",2*p/c);
    return 0;
}