#include <algorithm>
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define all(x) (x).begin(), (x).end()
#define ll long long
#define ld long double
#define INF 1000000000000000000
typedef pair<ll, ll> pll;

int main() {
    string s;
    cin >> s;
    vector<int> v, ans;
    int cnt = 0;
    rep(i, s.size()) {
        if (s[i] != '0')
            cnt += 1;
        else {
            if (cnt != 0)
                v.push_back(cnt);
            v.push_back(0), cnt = 0;
        }
    }

    if (cnt > 0)
        v.emplace_back(cnt);
    cnt = 0;
    rep(i, v.size()) {
        if (v[i] != 0)
            cnt += v[i];
        else if (i < v.size() - 1) {
            if (v[i + 1] != 0 && cnt + v[i + 1] >= 3)
                ans.push_back(1);
            else
                ans.push_back(cnt), cnt = 0;
        }
    }

    if (cnt > 0)
        ans.push_back(cnt);

    ll res = 0;
    rep(i, ans.size()) {
        if (ans[i] >= 2)
            res += 2;
        else
            res += ans[i];
    }

    cout << res << endl;
}