#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, h;
  cin >> n >> m >> h;
  int s[m];
  for (int i = 0; i < m; ++i) cin >> s[i];
  int sum = 0;
  for (int i = 0; i < m; ++i) sum += s[i];
  if (sum < n) {
    cout << -1.0;
    return 0;
  }
  double ans = 1;
  sum -= s[h - 1];
  int sum2 = sum + s[h - 1] - 1;
  if (sum < n - 1) {
    cout << 1;
    return 0;
  }
  cout.precision(10);
  for (int i = 1; i <= n - 1; ++i) {
    ans *= (sum - i + 1);
    ans /= (sum2 - i + 1);
  }
  cout << 1 - ans;
  return 0;
}
