#==================================================
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

    for a in range(A_max+2):
        if C[a]>=3:
            alpha=a
            break
    else:
        alpha=A_max

    C[alpha]-=1

    beta=0
    while C[beta]>=2:
        beta+=1

    C[beta]+=1

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

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