m=10**5+10
n=int(input())
a=list(map(int,input().split()))
mx=max(a)
a.remove(mx)
cnt=[0]*m
for i in a:
  cnt[i]+=1
if max(cnt)>=3:
  cnt[mx]+=1
use=0
flag=0
for i in range(m):
  if cnt[i]==0:
    print(i)
    exit()
  if cnt[i]==1:
    if flag:
      print(i)
      exit()
    else:
      flag=1