#include <bits/stdc++.h>
using namespace std;
int main() {
  long long i, j, k, p, q, m, n, x, y;
  cin >> n >> m >> k;
  x = 0, y = 0;
  for (i = 1; i <= m; i++) {
    cin >> p;
    x += p;
    if (i == k) y -= p;
  }
  if (x < n) {
    cout << -1 << endl;
    return 0;
  }
  y += x;
  x--;
  double d = 1.00;
  for (i = 1; i < n && y >= 0 && x >= 0; i++, y--, x--) {
    d *= (double)y / (double)x;
  }
  cout << fixed << setprecision(8) << 1.00 - d << endl;
  return 0;
}
