#include <bits/stdc++.h>
const long long MOD = 1000000007;
using namespace std;
double a[3000], s, sh, ans;
int n, m, h;
int main() {
  cin >> n >> m >> h;
  for (int i = 1; i <= m; i++) cin >> a[i];
  s = 0;
  sh = 0;
  for (int i = 1; i <= m; i++) sh += a[i];
  s = sh - a[h];
  if (sh < n) {
    cout << -1;
    return 0;
  }
  if (n == 1 || a[h] == 1) {
    cout << 0;
    return 0;
  }
  if (m == 1) {
    cout << 1;
    return 0;
  }
  if (s < n - 1) {
    cout << 1;
    return 0;
  }
  sh -= 1;
  ans = 1;
  for (int i = 2; i <= n; i++) ans *= (s - n + i) / (sh - n + i);
  printf("%.10lf", 1 - ans);
  return 0;
}
