#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
#define rep(a, b, c) for (int a = b; a < (int)c; ++a)

vector<int> xm = {-1, 1, 0, 0};
vector<int> ym = {0, 0, 1, -1};

const ll MOD = 998244353;

int main()
{
    string s;
    cin >> s;
    int ans = 0;
    rep(i, 0, s.size() - 2)
    {
        if (s[i] == '1' && s[i + 1] == '1' && s[i + 2] == '0')
        {
            ans++;
            s[i] = '0', s[i + 1] = '1', s[i + 2] = '1';
            i = max(0, i - 4);
        }
    }
    cout << ans << endl;
}