/* -*- coding: utf-8 -*-
 *
 * 2126.cc:  No.2126 MEX Game - yukicoder
 */

#include<cstdio>
#include<algorithm>
 
using namespace std;

/* constant */

const int MAX_N = 100000;
const int MAX_A = 100000;

/* typedef */

/* global variables */

int as[MAX_N], cs[MAX_A + 1];

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%d", as + i);

  for (int i = 0; i < n; i++) cs[as[i]]++;

  int k = 0, sum = n;
  while (cs[k] >= 2) {
    sum -= 2;
    k++;
  }
  //printf("cs[%d] = %d\n", k, cs[k]);

  if (cs[k] == 1) {
    sum--;
    int l = k + 1;
    while (cs[l] >= 2) {
      sum -= 2;
      l++;
    }
    if (sum == 0) l--;
    //printf("cs[%d] = %d, cs[%d] = %d\n", k, cs[k], l, cs[l]);
    printf("%d\n", l);
  }
  else
    printf("%d\n", k);
  return 0;
}