#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int l = s.size();
  int c0 = 0, c1 = 0;
  for (int i = 0; i < l; i++) {
    if (s[i] == '0')
      c0++;
    else
      c1++;
  }
  cout << min(c0, c1) << "\n";
  return 0;
}
