#include <bits/stdc++.h>
using namespace std;
int n, m, h, sum, a[1005];
int main() {
  cin >> n >> m >> h;
  for (int i = 1; i <= m; ++i) {
    cin >> a[i];
    sum += a[i];
  }
  --a[h], --sum;
  if (sum < n - 1) {
    cout << -1;
    return 0;
  }
  if (sum - a[h] < n - 1) {
    cout << 1;
    return 0;
  }
  double ans = 1.0;
  int k = sum - a[h];
  for (int i = 1; i <= n - 1; ++i) {
    ans *= (k * 1.0) / (sum * 1.0);
    --k, --sum;
  }
  cout << 1.0 - ans;
}
