#include <bits/stdc++.h>
using namespace std;

bool check(int x){
    int f = 1;
    while(x){
        if(x % 2 != f) return false;
        x /= 10;
        f = 1 - f;
    }
    return true;
}

int main(){
    int ans = 0;
    int n;
    cin >> n;
    for(int i = 1; i <= n; i += 2){
        if(check(i)) ans ++;
    }
    cout << ans;
}