#include <bits/stdc++.h>
using namespace std;
int main() {
  int m, h, i;
  double n, sum = 0.0, s[1111], p = 1.0;
  cin >> n >> m >> h;
  for (i = 1; i <= m; i++) {
    cin >> s[i];
    sum += s[i];
  }
  if (n > sum) {
    cout << "-1.0";
    return 0;
  }
  if (n - 1 > sum - s[h]) {
    cout << "1.0";
    return 0;
  }
  for (i = 1; i < n; i++) p *= (sum - s[h] - n + i + 1) / (sum - n + i);
  cout.precision(9);
  cout << fixed << 1 - p << endl;
  return 0;
}
