c = [0] * 100001
n = int(input())
for i in list(map(int, input().split())):
    c[i] += 1
one, aff = False, False
sum, k = 0, 0
while True:
    aff |= c[k] > 2
    sum += c[k]
    if c[k] == 0:
        print(k)
        exit()
    if c[k] == 1:
        if one:
            print(k)
            exit()
        one = True
        if sum == n and not aff:
            print(k)
            exit()
    k += 1