[백준] 1065

n = int(input())
d = [0, 1, 2, 3, 4, -1, -2, -3, -4]
cnt = 0
for i in range(1, n+1):
  s = str(i) 
  if i <= 99:
    cnt += 1
  else:
    d = int(s[1]) - int(s[0])
    t = 0 
    for i in range(len(s)-1):
      if int(s[i+1]) - int(s[i]) != d:
        t=1; break;
    if t==0:
      cnt += 1

print(cnt)