#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int n = s.size();
  int count0 = 0;
  int count1 = 0;
  for (int i = 0; i < n; i++) {
    if (s[i] == '0')
      count0++;
    else
      count1++;
  }
  cout << min(count0, count1) << endl;
}
