#include <iostream>
#include <fstream>
#include <iomanip>
#include <cstdio>
#define DN 30
using namespace std;

int n,t,ns=0;
double a[30],sum;
static inline void gen(int k, int b, double p) {
    if(k==t) {
        ++ns;
        sum+=p;
        return;
    }
    for (int i=b+1; i<=n+k-t+1; ++i) gen(k+1,i,p*a[i]);
}

int main()
{
    ifstream fin("dezastru.in");
    ofstream fout("dezastru.out");
    fin>>n>>t;
    for(int i=1;i<=n;i++)
        fin>>a[i];
    gen(0,0,1);
   fout<<(sum/double(ns));
    return 0;
}