from collections import defaultdict
N = int(input())
A = list(map(int,input().split()))
cnt = defaultdict(int)
for a in A:
    cnt[a] +=1
one = []
two = []
other = 0
for key in cnt.keys():
    if cnt[key] == 1:
        one.append(key)
    elif cnt[key] == 2:
        two.append(key)
    else:
        other +=1
setA = set(A)
for i in range(N+1):
    if i in setA:
        pass
    else:
        ans = i
        break
one.sort()
two.sort()
if len(one) >= 1:
    co = -1
    if len(one) >=2:
        co = one[1]
    if len(two) >= 1 and two[-1] > one[0]:
        co = max(co,two[-1])
    if co == -1:
        ans = min(ans,one[0])
    else:
        ans = min(ans,co)

else:
    pass
print(ans)