#include <iostream>
#include <fstream>
#include <bitset>
#include <iomanip>
using namespace std;
ifstream in("dezastru.in");
ofstream out("dezastru.out");
int n, k, chance;
double s, v[26];
bitset<26> u;
void read(){
    in>>n>>k;
    for(int i=1; i<=n; i++)in>>v[i];
}

void backtrack(int x, double val){
    if(x>k){
        s+=val;
        chance++;
        return;
    }
    for(int i=1; i<=n; i++){
        if(u[i]==0){
            double p;
            u[i]=1;
            if(x==1)p=v[i];
            else p=val*v[i];
            backtrack(x+1, p);
            u[i]=0;
        }
    }
}

void print(){
    out<<fixed<<setprecision(6)<<s/chance;
}

int main(){
    read();
    backtrack(1, 0);
    print();
    return 0;
}