#include <bits/stdc++.h>
using namespace std;
using ll = long long;

#ifdef LOCAL
#include <debug.hpp>
#else
#define debug(...)
#endif

vector<pair<char, int>> run_length_encodeing(const string& S) {
    vector<pair<char, int>> res;
    for (auto c : S) {
        if (!res.empty() && c == res.back().first) {
            res.back().second++;
        } else {
            res.emplace_back(c, 1);
        }
    }
    return res;
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
    string S;
    cin >> S;
    auto res = run_length_encodeing(S);

    int ans = 0;
    stack<pair<char, int>> stk;
    for (auto&& [c, cnt] : res) {
        if (!stk.empty() && stk.top().first == '1' && c == '0') {
            auto [c2, cnt2] = stk.top();
            ans += cnt2 / 2;
            cnt--;
        }
        if (stk.empty()) {
            stk.emplace(c, cnt);
        } else if (stk.top().first == c) {
            stk.top().second += cnt;
        } else if (cnt > 0) {
            stk.emplace(c, cnt);
        }
    }

    cout << ans << endl;
}