#include <bits/stdc++.h>
using namespace std;
long double d(long double n1, long double s, long double sms) {
  long double tmp = 1;
  for (int i = n1; i > 0; --i) tmp *= (s - i + 1) / (sms - i);
  return tmp;
}
int main() {
  int n, m, h;
  scanf("%d%d%d", &n, &m, &h);
  vector<int> a(m + 1);
  int sms = 0;
  for (int i = 1; i <= m; i++) {
    scanf("%d", &a[i]);
    sms += a[i];
  }
  int n1 = n - 1;
  int s = sms - a[h];
  if (sms < n) {
    cout << -1;
    return 0;
  }
  if (s < n1) {
    cout << 1;
    return 0;
  }
  long double ans = 1;
  cout << setprecision(10) << fixed << 1 - d(n1, s, sms);
  return 0;
}
