#include <bits/stdc++.h>
using namespace std;
int players[1050];
int main() {
  int n, m, h;
  cin >> n >> m >> h;
  --h;
  double sum = 0;
  for (int(i) = 0; (i) < (m); ++(i)) {
    cin >> players[i];
    if (i == h) --players[i];
    sum += (double)players[i];
  }
  --n;
  if (sum < n) {
    cout << "-1.0" << endl;
    return 0;
  }
  if (players[h] == 0 || n == 0) {
    cout << "0.0" << endl;
    return 0;
  }
  if (sum - players[h] < n) {
    cout << "1.0" << endl;
    return 0;
  }
  double antichance = 1.0;
  for (int i = 0; i < n; ++i) {
    antichance *= (double)(sum - players[h] - i) / (double)(sum - i);
  }
  printf("%.10lf\n", 1.0 - antichance);
  return 0;
}
