//#include "bits/stdc++.h"
#include <vector>
#include <algorithm>
#include <iostream>
#include <queue>

using namespace std;
#define ll long long
#define endl '\n'
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define rep2(i,a,b) for(int i=int(a);i<int(b);i++)
#define pb push_back
#define all(x) (x).begin(),(x).end()
int mod=1e9+7;
int mod2=998244353;
const int INF=1e9;

signed main(){
    ll n;
    cin>>n;
    ll ans=0;
    rep2(i,1,n+1){
        string s=to_string(i);
        if(s.size()%2==1)ans++;
    }
    cout<<ans<<endl;
}

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