#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<<c;
}
/**************************************************************
	Problem: 1729
	User: linzihang
	Language: C++
	Result: Accepted
	Time:27 ms
	Memory:2072 kb
****************************************************************/