#include <bits/stdc++.h>
using namespace std;
int n, m, h;
int sum, s;
int main() {
  while (cin >> n >> m >> h) {
    sum = 0;
    for (int i = 1, j; i <= m; i++) {
      cin >> j;
      sum += j;
      if (i == h) s = j;
    }
    if (sum < n) {
      cout << "-1.0" << endl;
      continue;
    }
    if (s == 1) {
      cout << "0.00" << endl;
      continue;
    }
    sum -= s;
    if (sum < n - 1 || sum + s == n) {
      cout << "1.0" << endl;
      continue;
    }
    double pp = 1.000000000000000;
    double a = (double)sum;
    double b = (double)sum + s - 1;
    for (int i = 1; i < n; i++) {
      pp *= a / b;
      a -= 1;
      b -= 1;
    }
    printf("%.10lf\n", 1.00000 - pp);
  }
}
