#include <bits/stdc++.h>
using namespace std;
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
  std::cerr << name << " : " << arg1 << '\n';
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
  const char* comma = strchr(names + 1, ',');
  std::cerr.write(names, comma - names) << " : " << arg1 << " | ";
  __f(comma + 1, args...);
}
int ans = 0;
signed main() {
  std::ios_base::sync_with_stdio(false);
  int n, m, h;
  cin >> n >> m >> h;
  vector<int> data;
  for (int i = 0; i < m; i++) {
    int tmp;
    cin >> tmp;
    data.push_back(tmp);
    ans += tmp;
  };
  if (ans < n) {
    cout << -1;
    return 0;
  }
  int alpha = ans - data[h - 1];
  int k = data[h - 1] - 1;
  if (alpha < n - 1) {
    cout << 1;
    return 0;
  }
  double prob = 1;
  for (int i = 0; i < n - 1; i++) {
    prob *= ((double)alpha - i) / (alpha + k - i);
  }
  prob = 1.0 - prob;
  cout << fixed << setprecision(9) << prob;
  return 0;
}
