#include <bits/stdc++.h>

using namespace std;

ifstream fin("dezastru.in");
ofstream fout("dezastru.out");

const int MAXN = 25;
long double p[1 + MAXN], dp[1 + MAXN][1 + MAXN];
int64_t C[1 + MAXN][1 + MAXN];

void test_case() {
  int n, k;
  fin >> n >> k;
  for (int i = 1; i <= n; ++i) {
    fin >> p[i];
    dp[i][1] = dp[i - 1][1] + p[i];
    C[i][0] = 1;
    C[i][1] = i;
  }
  for (int i = 2; i <= n; ++i) {
    for (int j = 2; j <= k; ++j) {
      dp[i][j] = dp[i - 1][j] + dp[i - 1][j - 1] * p[i];
      C[i][j] = C[i - 1][j] + C[i - 1][j - 1];
    }
  }
  fout << fixed << setprecision(12) << dp[n][k] / C[n][k] << '\n';
}

int main() {
  int t = 1;
  for (int tc = 1; tc <= t; ++tc) {
    test_case();
  }
  fin.close();
  fout.close();
  return 0;
}