#include<bits/stdc++.h>
#define ll long long int
using namespace std;
 
int fib(int n)
{
    if (n <= 1)
        return n;
    return fib(n-1) + fib(n-2);
}
 
int main ()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    ll n,tmp=0;
    cin>>n;
    ll tm=log10(n);
    tm/=2;
    tmp=pow(10,tm-1)-1;
    ll ans=tmp;
    tmp+=1;
    //cout<<ans<<" "<<tmp<<"\n";
    for(ll i=tmp;i<=tmp*100;i++)
    {
         ll y=(log10(i)+1);
         ll mul=pow(10,y);
        ll k=i*mul;
         k+=i;
         //cout<<k<<" ";
        if(k<=n)
        ans++;
        else
        break;
    }
    cout<<ans;
}

#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,fma")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h> 
#include <complex>
#include <queue>
#include <set>
#include <unordered_set>
#include <list>
#include <chrono>
#include <random>
#include <iostream>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <map>
#include <unordered_map>
#include <stack>
#include <iomanip>
#include <fstream>
 
using namespace std;
 
typedef long long ll;
typedef long double ld;
typedef pair<int,int> p32;
typedef pair<ll,ll> p64;
typedef pair<double,double> pdd;
typedef vector<ll> v64;
typedef vector<int> v32;
typedef vector<vector<int> > vv32;
typedef vector<vector<ll> > vv64;
typedef vector<vector<p64> > vvp64;
typedef vector<p64> vp64;
typedef vector<p32> vp32;
ll MOD = 998244353;
double eps = 1e-12;
#define forn(i,e) for(ll i = 0; i < e; i++)
#define forsn(i,s,e) for(ll i = s; i < e; i++)
#define rforn(i,s) for(ll i = s; i >= 0; i--)
#define rforsn(i,s,e) for(ll i = s; i >= e; i--)
#define ln "\n"
#define dbg(x) cout<<#x<<" = "<<x<<ln
#define mp make_pair
#define pb push_back
#define fi first
#define se second
#define INF 2e18
#define fast_cin() ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL)
#define all(x) (x).begin(), (x).end()
#define sz(x) ((ll)(x).size())
 
void solve(){
}
int main()
{
    fast_cin();
    ll n,sum=0;
    string s;
    int max,fh,sh;
    cin >> n;
    s = to_string(n);
    int len = s.length();
    if(len == 1) {
        cout << 0;
    }
    else if(len % 2 == 1) {
        for(int i=1; i<=len/2; i++) {
            cout << 9;
        }
    }
    else {
        fh = stoi(s.substr(0,len/2));
        sh = stoi(s.substr(len/2));
        if(fh <= sh) {
            cout << fh;
        }
        else {
            cout << fh-1;
        }
    }
}