#include <bits/stdc++.h>
using namespace std;
const int N = 1005;
int n, m, h, s[N];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> m >> h;
  int sum = 0;
  for (int i = 1; i <= m; i++) {
    cin >> s[i];
    sum += s[i];
  }
  s[h]--;
  sum--;
  n--;
  if (sum < n) {
    return cout << "-1", 0;
  }
  if (sum - s[h] < n) {
    return cout << "1", 0;
  }
  double ans = 1;
  for (int i = 1; i <= n; i++) {
    ans *= (double)(sum - s[h] - n + i) / (1.0 * (sum - n + i));
  }
  cout << fixed << setprecision(10) << 1 - ans;
}
