N = int(input())
A = list(map(int, input().split()))
A.sort()
indl = 0
indr = N
M = 10**5
for i in range(M):
    ct = 0
    while indl < indr and A[indl] == i:
        ct += 1
        indl += 1
    if ct == 0:
        print(i)
        exit()
    if ct == 1:
        if indl == indr:
            print(i)
            exit()
        else:
            indr -= 1