#include <bits/stdc++.h>

#define NMax 26
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");

double a[NMax];
double dp[NMax][NMax];
double sum;
int n,k;

int main()
{
    f >> n >> k;
    for(int i = 1; i <= n; ++i)
        f >> a[i];
    for(int i = 1; i <= n; ++i){
        dp[1][i] = a[i];
        for(int j = n - k + 1; j <= n; ++j){
            dp[1][i] /= j;
        }
    }
    for(int i = 2; i <= n; ++i){
        for(int j = i; j <= n; ++j){
            for(int k = 1; k < j; ++k){
                dp[i][j] += (a[j] * dp[i - 1][k]);
            }
        }
    }
    for(int i = k; i <= n; ++i){
        sum += dp[k][i];
    }
    for(int i = 1; i <= k; ++i){
        sum *= i;
    }
    g << fixed << setprecision(6) << 1.0*sum << '\n';
    return 0;
}