n = int(input())
a = list(map(int,input().split()))
from collections import Counter
c = Counter(a)
if c[0] == 0:
  print(0)
else:
  ans = 0
  amari = 0
  flg = True
  now = []
  for i in range(n+10):
    if c[i] >= 2:
      now.append(c[i])
    elif c[i] == 1 and flg:
      now.append(c[i])
      flg = False
    else:
      break
  flg = False
  for i in now:
    if i >= 3:
      flg = True
  if flg:
    print(len(now))
  else:
    print(len(now)-1)