#include <bits/stdc++.h>
using namespace std;
string s;
int k;
int main() {
  cin >> s;
  for (int i = 0; i < s.length(); i++)
    if (s[i] == '1') k++;
  if (k > 1)
    cout << (s.length() + 1) / 2 << endl;
  else
    cout << s.length() / 2 << endl;
  return 0;
}
