from typing import List, Tuple, Callable, TypeVar
import sys
import itertools
import heapq
import bisect
import math
from collections import deque, defaultdict, Counter
from functools import lru_cache, cmp_to_key

input = sys.stdin.readline

if __file__ != 'prog.py':
    sys.setrecursionlimit(10 ** 6)


def readints(): return map(int, input().split())
def readlist(): return list(readints())
def readstr(): return input()[:-1]


N = int(input())
A = readlist()

cnt = defaultdict(int)
for a in A:
    cnt[a] += 1

for i in range(10 ** 5 + 10):
    if cnt[i] <= 1:
        print(i)
        break