#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;
int factorial(int x){
    int p=1;
    for(int i=1; i<=x; i++)p*=i;
    return p;
}

void read(){
    in>>n>>k;
    for(int i=1; i<=n; i++)in>>v[i];
    chance=factorial(n);
}

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

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

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