#include <stdio.h>

int main () {
  int n = 0;
  int a = 0;
  
  int res = 0;
  
  int rem = 0;
  int cnt[100001] = {};
  
  int ans = 0;
  
  res = scanf("%d", &n);
  for (int i = 0; i < n; i++) {
    res = scanf("%d", &a);
    cnt[a]++;
  }
  
  rem = n;
  while (cnt[ans] > 1) {
    rem -= 2;
    ans++;
  }
  
  if (cnt[ans] > 0 && rem > 1) {
    ans++;
    rem -= 1;
  }
  
  while (cnt[ans] > 1 && rem > 2) {
    rem -= 2;
    ans++;
  }
  
  printf("%d\n", ans);
  return 0;
}