#include <iostream>
using namespace std;
using ll = long long;

int main() {
	int res = 0;
	int n = 0;
	cin >> n;
	
	for (int i = 1; i <= n; i += 2) {
		bool tag = 1;
		int x = i;
		while (x) {
			if ((x & 1) ^ tag)
				goto End;
			
			tag ^= 1;
			x /= 10;
		}
		++res;
	End:
		;
	}
	
	cout << res << '\n';
	return 0;
}