#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
ll N;
int cnt = 0;
void solve(ll num) {
	int tmp;
	int index = 0;
	while(num>0) {
		index++;
		tmp = num%10;
		if(index%2==1&&tmp%2!=1) return;
		if(index%2==0&&tmp%2!=0) return;
		num = num/10;
	}
	cnt++;
	return;
}
int main(){
	ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
	cin>>N;
	for(ll i = 1;i<=N;i++)
		solve(i);
	cout<<cnt;
	return 0;
}