#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long n, m, h;
  cin >> n >> m >> h;
  vector<long long> v(m);
  long long total = 0;
  for (long long i = 0; i < m; i++) {
    cin >> v[i];
    total += v[i];
  }
  v[h - 1]--;
  n--;
  total--;
  if (total < n) {
    cout << -1;
    return;
  }
  if ((total - v[h - 1]) < n) {
    cout << 1;
    return;
  }
  long long numerator = total - v[h - 1];
  double ans = 1;
  for (long long i = 0; i < n; i++) {
    ans = ans * double(numerator - i) / double(total - i);
  }
  cout << 1 - ans << '\n';
}
int32_t main() {
  long long t;
  t = 1;
  while (t--) {
    solve();
  }
}
