import sys
input = sys.stdin.readline
from collections import *

N = int(input())
A = list(map(int, input().split()))
cnt = Counter(A)
need = 0

for i in range(10**5+10):
    if cnt[i]>=2:
        need += 2
    elif cnt[i]==1:
        if need+1==N:
            print(i)
            break
        else:
            print(i+1)
            break
    else:
        print(i)
        break