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
g = 0

for i in range(100002):
	if mex[i] == 1:
		if mode == 1:
			for j in range(i):
				if mex[j] >= 3:
					mex[j] -= 1
					mex[i] += 1
					mode = 0
			if mode:
				if mex[j] <= 1: g = 1
				for j in range(i+1, 100002):
					if mex[j] >= 3 or (g == 1 and mex[j] >= 1):
						mex[j] -= 1
						mex[i] += 1
						mode = 0
			if mode:
				if max(a) == i:
					print(i)
					exit()
				else:
					mex[max(a)] -= 1
					mex[i] += 1
					mode = 0
		else:
			print(i)
			exit()
	if mex[i] == 0:
		print(i)
		exit()