#include <stdio.h>
#include <algorithm>
using namespace std;
float v[26],m,p;
int i,n,k,perm[26],q;
int main()
{
    FILE *f1,*f2;
    f1=fopen("dezastru.in","r");
    f2=fopen("dezastru.out","w");
    fscanf(f1,"%d%d",&n,&k);
    for(i=1;i<=n;i++)
    {
        fscanf(f1,"%f",&v[i]);
        perm[i]=i;
    }
    do
    {
        p=1;
        for(i=1;i<=k;i++)
            p*=v[perm[i]];
        m+=p;q++;
    }while(next_permutation(perm+1,perm+n+1));
    fprintf(f2,"%f",m/q);
    return 0;
}