from collections import Counter

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

for i in range(n):
    if counter[i] >= 2:
        continue
    if counter[i] == 1:
        add = 0
        for j in range(i + 1, n):
            if counter[j] > 0:
                add = 1
                break
        print(i + add)
        break
    print(i)
    break