#include <bits/stdc++.h>
using namespace std;
bool is_integer(long double k) { return std::floor(k) == k; }
int main() {
  long long i, j = 0;
  string s;
  cin >> s;
  if (s.length() == 1 && s[0] == '0') {
    cout << 0;
    return 0;
  }
  int c = 0;
  for (i = (s.length() - 1); i >= 0; i--) {
    if (s[i] == '1') {
      c++;
    }
  }
  if (c == 1) {
    cout << floor(s.length() / 2.0);
  } else {
    cout << ceil(s.length() / 2.0);
  }
}
