#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();
    ll ans = 0;
    int count = 0;
    for (int i = 0; i < n; ++i) {
        if (s[i] == '1') count++;
        else {
            if (s[i] == '0') {
                ans += count / 2;
                count -= count % 2;
            }
        }
    }
    std::cout << ans << std::endl;
}