#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;
int n, res;
int check(int x)
{
	int cnt = 1;
	while(x)
	{
		if(cnt % 2 != x % 10 % 2) return 0;
		x /= 10;
		cnt ++;
	}
	return 1;
}
int main()
{
	cin >> n;
	for(int i = 1; i <= n; ++ i) if(check(i)) res ++;
	
	cout << res << '\n';
    return 0;
}