n = int(input())
inf = 10 ** 5 + 1
c = [0] * inf
for i in list(map(int, input().split())):
    c[i] += 1
one, ok1, ok2 = 0, True, 0
for i in range(inf):
    if c[i] >= 1:
        ok2 = i
k = 0
while 1:
    ok1 &= c[k] <= 2
    if c[k] == 1:
        one += 1
    if c[k] == 0 or one > 1 or one == 1 and ok1 and k >= ok2:
        print(k)
        exit()
    k += 1