#include <bits/stdc++.h>
using namespace std;
signed main() {
  int n, m, h;
  cin >> n >> m >> h;
  std::cout << std::fixed;
  std::cout << std::setprecision(10);
  int s[m];
  long long w = 0;
  long long r = 0;
  for (int i = 0; i < m; ++i) {
    cin >> s[i];
    if (i == h - 1)
      r += s[i] - 1;
    else
      w += s[i];
  }
  if (w + r + 1 < n) {
    cout << -1;
    return 0;
  }
  if (r == 0) {
    cout << 0;
    return 0;
  }
  if (w < n - 1) {
    cout << 1;
    return 0;
  }
  long double x = 1;
  for (int i = 0; i < n - 1; ++i) {
    x = x * (w - i) / (w + r - i);
  }
  cout << 1 - x;
}
