import collections
N = int(input())
A = list(map(int, input().split())) 
CA = collections.Counter(A)
ans = 0
amari = False
for k,v in CA.items():
    if v>=3:
        amari=True
while CA[ans]>=2:
        ans+=1
if CA[ans] and ans*2+1<N:
    ans+=1
    if not amari:
        CA[max(A)]-=1
while CA[ans]>=2:
        ans+=1
print(ans)