#include <bits/stdc++.h>

int main() {
	std::string s;
	std::cin >> s;
	std::reverse(s.begin(), s.end());
	int n = s.size();
	int res = 0;
	for (int i = 0; i < n; i++) {
		if (s[i] == '0') continue;
		int j = i;
		int cost = 1;
		while (j > 1) {
			if (s[j - 1] == '1' && s[j - 2] == '1') {
				cost = 0;
				break;
			}
			if (s[j - 1] == '1' && s[j - 2] == '0') j -= 2;
			else break;
		}
		res += cost;
	}
	std::cout << res << std::endl;
	return 0;
}