#include <bits/stdc++.h>
using namespace std;
int s[1005];
int main() {
  int n, m, h;
  cin >> n >> m >> h;
  h--;
  long long all = 0, others = 0;
  for (int i = 0; i < m; i++) {
    cin >> s[i];
    all += s[i];
    if (i != h) others += s[i];
  }
  all--;
  if (all < n - 1) {
    cout << "-1.0\n";
    return 0;
  } else if (others < n - 1) {
    cout << "1.0\n";
    return 0;
  }
  double ans = 1;
  for (int i = 1; i <= all - others; i++)
    ans *= double((others - n + 1) + i) / (others + i);
  printf("%0.16f\n", 1 - ans);
  return 0;
}
