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
  flg = True
  for i in range(n+10):
    if c[i] >= 2:
      ans += 1
      amari += c[i] - 2
    else:
      if flg and amari >= 1 and c[i] == 1:
        ans += 1
        flg = False
      else:
        break

  print(ans)