from collections import Counter
n = int(input())
A = list(map(int,input().split()))
A.sort()
if A[0] != 0:
    print(0)
    exit()

C = Counter(A)

s = sorted(C.keys())

more = 0
done = 0
last = -1
for x in s:
    if last+1 != x:
        print(last+1)
        exit()
    last = x
    if C[x] == 2:
        continue
    if C[x] > 2:
        more = 1
        continue

    if x != max(s) or more:
        if done:
            print(x)
            exit()
        else:
            done = 1
    else:
        print(x)
        exit()


print(last+1)