题解 | #自守数#

import sys

while True:
    try:
        nums = 0
        n = int(input())
        for i in range( 0 , n + 1 ):
            if i <= 9:
                len = 1
            elif i <= 99 :
                len = 2
            elif i<= 999:
                len = 3
            else :
                len = 4

            if i ** 2 % 10**len == i :
                nums += 1
        print(nums)
    except:
        break