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