#include <iostream>
#include <fstream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <cstring>
#include <string>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <algorithm>
using namespace std;

#define REP(i,n) for(int i=0; i<n; ++i)
#define FOR(i,a,b) for(int i=a; i<=b; ++i)
#define FORR(i,a,b) for (int i=a; i>=b; --i)
#define ALL(c) (c).begin(), (c).end()

typedef long long ll;
typedef vector<int> VI;
typedef vector<ll> VL;
typedef vector<VI> VVI;
typedef pair<int,int> P;
typedef pair<ll,ll> PL;

ll powll(ll x, ll y){
    if (y == 0) return 1;
    return x * powll(x, y-1);
}

int main() {
    ll n;
    cin >> n;
    int ans = 0;
    FOR(i,1,9){
        int lim = powll(10,(i+1)/2);
        FOR(p,1,lim-1){
            if (p % 10 == 0) continue;
            ll x = p, y = p;
            REP(a,i/2){
                x += (y%10)*powll(10,i-a-1);
                y /= 10;
            }
            // cout << x << endl;
            x += x*powll(10,9);
            // cout << x<< endl;
            if (x <= n){
                ans++;
            }                
        }
    }
    cout << ans << endl;
    return 0;
}