#include <bits/stdc++.h>

typedef long long ll;
typedef std::pair<long long, long long> P;
typedef std::priority_queue<P, std::vector<P>, std::greater<P>> PQ;

int main()
{
    std::string s;
    std::cin >> s;
    int n = s.length();
    int ans = 0;
    for (int i = n - 1; i >= 2; --i) {
        if (s[i] == '0' && s[i - 1] == '1' && s[i - 2] == '1') {
            s[i] = '1';
            s[i - 2] = '0';
            ans++;
            i += 2;
            if (i > n) i = n;
            // std::cout << s << std::endl;
        }
    }
    std::cout << ans << std::endl;
}