#include <bits/stdc++.h>
using namespace std;
int n, m, h, cnt, a[1005];
double solve(int x, int y, int r) {
  double ret = 1;
  while (r--) {
    ret *= (x-- * 1.0 / y--);
  }
  return ret;
}
int main() {
  cin >> n >> m >> h;
  for (int i = 1; i <= m; ++i) {
    cin >> a[i];
    cnt += a[i];
  }
  if (cnt < n) {
    return cout << -1, 0;
  }
  printf("%.09lf", 1 - solve(cnt - n, cnt - 1, a[h] - 1));
  return 0;
}
