#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int N, K, el;
long long p1 = 1, p2 = 1;
long double a[30][30], p = 1, sum, sol, v[30];
int main()
{
    f >> N >> K;
    for(int i = 1; i <= N; i++){
        f >> v[i];
        a[i][1] = v[i] + a[i - 1][1];
    }
     for(int i = 1; i <= N; ++i)
        for(int j = 2; j <= N; ++j)
        a[i][j] = a[i - 1][j] + a[i - 1][j - 1] * v[i];
     sol = a[N][K];
     if(K > N / 2) K = N - K;
     for(int i = 1; i <= K; i++){
        p1 *= N - i + 1;
        p2 *= i;
     }
     el = p1 / p2;
     sol /= el;
     g << fixed << setprecision(6) << sol;
    return 0;
}