#include <bits/stdc++.h>
using namespace std;
const int MAXM = 1010;
int s[MAXM];
int main() {
  ios::sync_with_stdio(false);
  cout.tie(0);
  cin.tie(0);
  int n, m, h;
  cin >> n >> m >> h;
  h--;
  int f, sum = 0;
  for (int i = 0; i < m; i++) {
    cin >> s[i];
    if (h != i)
      sum += s[i];
    else
      f = s[i] - 1;
  }
  cerr << sum << ", " << f << ", " << n - 1 << endl;
  if (n - 1 > sum + f) {
    cout << -1;
    return 0;
  }
  if (n - 1 > sum) {
    cout << 1;
    return 0;
  }
  double ans = 1;
  for (int i = 0; i < n - 1; i++) ans *= double(sum - i) / (sum + f - i);
  cout << fixed << setprecision(10) << 1 - ans << endl;
}
