# https://www.acmicpc.net/problem/1065
def HanSoo(num):
# 한,두자리숫 자이면 무조건 한수
if 0 <= num // 10 <= 9:
return True
else:
baek = num // 100
sip = (num//10)% 10
ill = num%10
if (baek-sip) == (sip - ill):
return True
else:
return False
# 숫자 입력
n = int(input())
count = 0
for i in range(1, n+1):
if HanSoo(i):
count += 1
print(count)