#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int a, b;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] == '0') {
      a++;
    } else {
      b++;
    }
  }
  cout << min(a, b) << endl;
}
