n = int(input())
a = list(map(int,input().split()))

# 0, 0, 1, 2, 2, 3, 3
# -> 0, 0, 1, 1, 2, 2, 3
# の感じで1回だけ2にして安定させられる
# あとはまねっこ

mex = [0] * 100002
for i in range(n):
	mex[a[i]] += 1

mode = 1
mx = max(a)

for i in range(100002):
	if mex[i] == 1:
		if mode == 1:
			if mx == i:
				print(i)
				exit()
			mex[mx] -= 1
			mex[i] += 1
			mode = 0
		else:
			print(i)
			exit()
	if mex[i] == 0:
		print(i)
		exit()