#include <bits/stdc++.h>
using namespace std;

int i,n,c = 0;
int main() {
	 cin>>n;
	 for(i = 1;i <= n;i++){
	 	if(i*i%10==i||i*i%100==i||i*i%1000==i||i*i%10000==i||i*i%100000==i||i*i%1000000==i){
	 		c++;
	 		//cout<<i<<endl;
		 }
	 }
	 
	 cout<<c;
}


/**************************************************************
	Problem: 1729
	User: admin
	Language: C++
	Result: Accepted
	Time:47 ms
	Memory:2072 kb
****************************************************************/