#include <bits/stdc++.h>
using namespace std;
long long int C(long long int n, long long int r) {
  if (n < r) {
    return 0;
  }
  long long int num = 1;
  for (int i = 1; i <= r; i++) {
    num = num * (n + 1 - i) / i;
  }
  return num;
}
int main() {
  long long int n, m, h, sum = 0, num;
  cin >> n >> m >> h;
  vector<long long int> s(m + 1);
  for (int i = 1; i <= m; i++) {
    cin >> num;
    s[i] = num;
    sum += num;
  }
  if (sum < n) {
    cout << -1 << endl;
  } else {
    double a = sum - 1, b = sum - s[h], res = 1;
    if (b < n - 1) {
      res = 0;
    } else {
      for (long long int i = 1; i <= n - 1; i++) {
        res = res * (b + 1 - i) / (a + 1 - i);
      }
    }
    cout << std::setprecision(8) << (1 - res) << endl;
  }
}
