#include <bits/stdc++.h>
int main() {
  std::string S;
  std::cin >> S;
  const int Red = '0';
  const int Blue = '1';
  int redCount = 0;
  int blueCount = 0;
  for (auto i = S.crbegin(); i != S.crend(); i++) {
    if (*i == Red) {
      redCount++;
    } else {
      blueCount++;
    }
  }
  std::cout << (std::min)(redCount, blueCount) / 2 << std::endl;
  return 0;
}
