def mex(S):
    x=0
    while x in S:
        x+=1
    return x

def solve():
    from collections import Counter

    N=int(input())
    A=list(map(int,input().split()))
    CA=Counter(A)

    S=set()
    for a in CA:
        if CA[a]>=2:
            S.add(a)
    return mex(S)

#==================================================
print(solve())