#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;
ifstream in("dezastru.in");
ofstream out("dezastru.out");
double atacuri[30];
int v[55],n,k;
int main()
{
    in>>n>>k;
    for(int i=1;i<=n;++i){
        in>>atacuri[i];
    }
    for(int i=1;i<=n;++i){
        v[i]=v[i+n]=v[i+2*n]=i;
    }
    int nnn=2*n+k-1;
    double final=0;
    for(int i=1;i<=nnn;i++) {
        double faza=1.00;

        for(int j=1;j<=k;j++){
            faza *= atacuri[v[i-j+1]];

        }
        final += faza;


    }out<<setprecision(6)<<final/(2*n)<<endl;
    return 0;
}