#include<fstream>
#include<algorithm>
using namespace std;
double a[27],s,p;
int n,nr,k;
ifstream in("dezastru.in"); ofstream out("dezastru.out");
int main(){
    in>>n>>k;
    for(int i=1;i<=n;++i)
        in>>a[i];
    do{
        p=1; ++nr;
        for(int i=1;i<=k;++i)
            p*=a[i];
        s+=p;
    }while(next_permutation(a+1, a+n+1));
    out<<s/(double)nr<<'\n'; out.close(); return 0;
}