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

int main(){
	int n,ans=0;
	cin>>n;
	for(int i=1;i<=n;i+=2){
		string s=to_string(i);
		int len=s.size();
		int k=0;
		for(int i=len-1;i>=0;i--,++k){
			if((s[i]%2!=0&&k%2==0)||(s[i]%2==0&&k%2!=0)) continue;
			else break;
			//TO
		}
		if(k==len) ans++;
	}
	printf("%d",ans);
	return 0;
}