from collections import Counter

n = int(input())
A = list(map(int, input().split()))
counter = Counter(A)

more2_num = 0
for i in range(10**5 + 1):
    if counter[i] > 2:
        more2_num += 1
for i in range(10**5 + 1):
    if counter[i] >= 2:
        continue
    if counter[i] == 1:
        if more2_num > 0:
            more2_num -= 1
            continue
        print(i)
        break
    print(i)
    break