import sys
input = sys.stdin.readline
from collections import Counter

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

C=Counter(sorted(A))

B=[C[i] for i in range(10**5)]

ind=-1
for i in range(10**5):
    if B[i]==1:
        ind=i
        break

for i in range(10**5):
    if B[i]>2 and ind!=-1:
        B[i]-=1
        B[ind]+=1
        ind=-1

for i in range(10**5-1,-1,-1):
    if B[i]>=1 and ind!=-1 and i>ind:
        B[i]-=1
        B[ind]+=1
        break

for i in range(10**5):
    if B[i]<2:
        print(i)
        break