#include <iostream>
#include <stdio.h>
#include <cstring>
#include <math.h>
#include <algorithm>
#include <vector>
#define rep(i,n) for(int i=0;i<n;i++)
#define FOR(i,a,b) for(int i=a;i<b;i++)
#define ll long long
#define LL long long
#define mass 1000000007
using namespace std;
int keta(int n,int k){return (n/int(pow(10,double(k-1)))) % 10;} //nの下からk桁目

int main(){
	
	ll n;
	cin>>n;
	int c=0;
	int m=n/1000000001;
	//1~mにいくつあるか探す
	if(m==0){
		cout<<0<<endl;
		return 0;
	}

	int mk=log10(double(m))+1;
	if(mk%2==0){
		int a=m/int(pow(10,double(mk/2)));
		c+=a-pow(10,double(mk/2)-1);
		int ga=0;
		rep(i,mk/2){
			ga+=keta(a,mk/2-i)*pow(10,double(i));
		}
		int b=m%int(pow(10,double(mk/2)));
		if(b>=ga){
			c++;
		}
	}else{
		int a=m/int(pow(10,double(mk/2)));
		c+=a-pow(10,double(mk/2));
		int ga=0;
		rep(i,mk/2){
			ga+=keta(a,mk/2-i+1)*pow(10,double(i));
		}
		int b=m%int(pow(10,double(mk/2)));
		if(b>=ga){
			c++;
		}
	}
	switch(mk-1){
		case 8:c+=9000;
		case 7:c+=9000;
		case 6:c+=900;
		case 5:c+=900;
		case 4:c+=90;
		case 3:c+=90;
		case 2:c+=9;
		case 1:c+=9;
		case 0:;
	}
	cout<<c<<endl;

	return 0;
	

}