#include<iostream>
using namespace std;

bool check(int a)
{
    int w=1;
    while(a)
    {
        int r=a/10;
        if(a%2!=w%2) return false;
        a/=10,w++;
    }
    return true;
}

int main()
{
    int n;
    cin>>n;
    int res=0;

    for (int i = 1; i <= n; i ++ )
    {
        if(check(i)) res++;
    }
    cout<<res<<endl;
    return 0;

}