#include <iostream>
#include <fstream>
#define MAX 25

using namespace std;

int main()
{
    ifstream in("dezastru.in");
    ofstream out("dezastru.out");
    int nr, comb, i;
    float arr[MAX];
    in >> nr >> comb;
    for (i = 0 ; i != nr; i++){
        in >> arr[i];
    }

    float sum = 0;
    int temp = 0,val = 0;
    while(temp != nr-comb+1){
        float inm = 1;
        for (i = temp ; i != temp + comb; i++){
            inm *= arr[i];
        }
        val++;
        sum+=inm;
        for (i = comb +temp; i!= nr; i ++){
            inm = inm/arr[i - 1] *arr[i];
            val++;
            sum+=inm;
        }
        temp++;
    }
    out << sum/val<<endl;
    return 0;
}