#include <bits/stdc++.h>
using namespace std;
const int MAX = 1005;
int N, M, H, s[MAX];
double di(int a, int b) {
  double ans = 1;
  for (int i = 0; i < N - 1; i++) {
    ans *= (double)(a - i) / (double)(b - i);
  }
  return ans;
}
int main() {
  cin >> N >> M >> H;
  H--;
  int total = 0;
  for (int i = 0; i < M; i++) {
    cin >> s[i];
    total += s[i];
  }
  if (total < N) {
    cout << "-1.0\n";
    return 0;
  }
  total--;
  s[H]--;
  double ans = 1 - di(total - s[H], total);
  printf("%.9f\n", ans);
}
