#include <bits/stdc++.h>
using namespace std;
const int N = 25;
int main(int argc, char *argv[]) {
  int n, m, h;
  cin >> n >> m >> h;
  int tot = 0;
  int x, sh;
  for (int i = (1), _b = (m + 1); i < (_b); ++i) {
    cin >> x;
    tot += x;
    if (i == h) sh = x;
  }
  if (tot < n) {
    cout << -1 << endl;
    return 0;
  }
  --tot, --sh;
  double ans = 1.0;
  for (int i = (0), _b = (sh); i < (_b); ++i)
    ans *= 1.0 * (tot - i - (n - 1)) / (tot - i);
  cout << 1.0 - ans << endl;
  return 0;
}
