n = int(input())
a = list(map(int,input().split()))
from collections import Counter
c = Counter(a)
if c[0] < 2:
  print(0)
else:
  ans = 0
  amari = 0
  for i in range(n+10):
    if c[i] >= 2:
      ans += 1
      amari += c[i] - 2
    else:
      break
  if c[ans] == 1 and amari >= 1:
    ans += 1
  print(ans)