#include <bits/stdc++.h>
using namespace std;
int main() {
  int i, temp, fix, n, m, h;
  cin >> n >> m >> h;
  int sum = 0;
  for (i = 1; i <= m; i++) {
    cin >> temp;
    sum += temp;
    if (i == h) fix = temp;
  }
  if (sum < n) {
    cout << "-1.0" << endl;
    return 0;
  } else if (fix == 1) {
    cout << "0.0" << endl;
    return 0;
  } else if (sum == n) {
    cout << "1.0" << endl;
    return 0;
  }
  if (sum - fix < n - 1) {
    cout << "1.0" << endl;
    return 0;
  }
  --sum;
  --n;
  if (sum - n > n)
    temp = n;
  else
    temp = sum - n;
  double up = 1, down = 1;
  for (i = 1; i <= temp; i++) {
    down *= i;
    up *= (sum + 1 - i);
  }
  double DOWN = up / down;
  sum = sum - fix + 1;
  if (sum - n > n)
    temp = n;
  else
    temp = sum - n;
  double UP = 1;
  up = 1, down = 1;
  for (i = 1; i <= temp; i++) {
    down *= i;
    up *= (sum + 1 - i);
  }
  UP = up / down;
  UP /= DOWN;
  cout << 1 - UP << endl;
  return 0;
}
