#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, h;
  cin >> n >> m >> h;
  int a = 0;
  int b = 0;
  int c = n - 1;
  int current;
  for (int i = 0; i < m; i++) {
    cin >> current;
    if (i == h - 1) {
      a += current - 1;
    } else {
      b += current;
    }
  }
  if (c > a + b) {
    cout << -1.0 << endl;
    return 0;
  }
  double result = 1.0;
  for (int i = a; i >= 1; i--) {
    result *= double(i + b - c) / double(i + b);
  }
  result = 1.0 - result;
  cout << fixed << setprecision(15) << result << endl;
  return 0;
}
