n = int(input())
A = list(map(int, input().split()))
from collections import defaultdict

count = defaultdict(int)
for a in A:
    count[a] += 1

mex = 0
while True:
    if count[mex] == 0:
        break
    mex += 1

ans = mex
for x in range(mex):
    if count[x] == 1:
        ans = x
        break

print(ans)