# 接收输入：思路是转换为字符串来处理
n = int(input())
c = 0
for i in range(1, n + 1):
    # 平方的结果转成字符串
    a = str(i * i)
    # 把i转化成字符串
    b = str(i)
    # 得到i字符串的位数
    t = len(b)
    if a[t*(-1):] == b:
        c += 1
print(c)
/**************************************************************
	Problem: 1729
	User: admin
	Language: Python
	Result: Accepted
	Time:359 ms
	Memory:34480 kb
****************************************************************/