#include<fstream>
using namespace std;
ifstream in ("dezastru.in");
ofstream out ("dezastru.out");
double v[30],d[30][30],sol,aux;
int n,k;
int main (void) {
    in >> n >> k;
    for (int i = 1; i <= n; i ++) {
        in >> v[i];
    }
    for (int i = 1; i <= n; i ++) {
        d[1][i] = v[i];
    }
    for (int i = 2; i <= k; i ++) {
        for (int j = i; j<= n; j ++) {
            aux = 0;
            for (int h = i-1; h <= j-1; h ++) {
                aux += d[i-1][h];
            }
            d[i][j] = aux * v[j];
        }
    }
    for (int j = k; j <= n; j ++) {
        sol += d[k][j];
    }
    for (int i = 1; i <= k; i ++) {
       sol *= i;
    }
    for (int i = 1; i <= n-k; i ++) {
        sol *= i;
    }
    for (int i = 1; i <= n; i ++) {
        sol /= i;
    }
    out << sol;
    return 0;
}