#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, h;
  cin >> n >> m >> h;
  --n, --h;
  int a[m], sum = 0, total = 0;
  for (int i = 0; i < m; ++i) {
    cin >> a[i];
    if (i == h) --a[i];
    if (i != h) sum += a[i];
    total += a[i];
  }
  if (sum < n && sum + a[h] >= n) {
    cout << 1;
    return 0;
  }
  if (sum + a[h] < n) {
    cout << -1;
    return 0;
  }
  double p = 1;
  for (int i = 0; i < n; ++i) {
    p *= (double)sum / (double)total;
    --sum;
    --total;
  }
  cout.setf(ios::fixed);
  cout.precision(10);
  cout << 1 - p;
  cin >> n;
  return 0;
}
