#==================================================
def solve():
    from collections import Counter

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

    A_max=max(A)
    C=[0]*(max(A)+2)
    for a in A:
        C[a]+=1

    C[A_max]-=1
    target=0
    while C[target]<=1:
        C[target]+=1
        break

    mex=0
    while C[mex]>=2:
        mex+=1
    return mex

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