from collections import Counter
n = int(input())
a = list(map(int, input().split()))
c = Counter(a)
sm = 0
flg = 1
for i in range(1<<20):
  sm += c[i]
  if c[i] < 2:
    if sm < n and c[i] and flg:
      flg = 0
      continue
    print(i)
    break