#include <bits/stdc++.h>
using namespace std;
long n, m, h, ts, calc[2000];
double p2, p1;
double gc(long n, long m) {
  if (m > n) return 0;
  double res = 0;
  for (int i = n; i > n - m; i--)
    res = res + log(i + .0), res = res - log(.0 + n - i + 1);
  return exp(res);
}
int main() {
  ios_base::sync_with_stdio(0);
  cin >> n >> m >> h;
  for (int i = 1; i <= m; i++) {
    cin >> calc[i];
    ts += calc[i];
  }
  if (ts < n) {
    cout << -1 << endl;
    return 0;
  }
  if (n == 1) {
    cout << 0 << endl;
    return 0;
  }
  if (calc[h] == 1) {
    cout << 0 << endl;
    return 0;
  }
  --n;
  --calc[h];
  --ts;
  double ans = 1.0;
  for (int i = 1; i <= n; i++)
    ans = ans * (ts - calc[h] - i + 1) / (ts - i + 1);
  cout << fixed << 1.0 - ans << endl;
  cin.get();
  cin.get();
  return 0;
}
