import java.util.*;
 
class Main {
 
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String s = sc.nextLine();
    
    int ans1 = 0;
    int ans2 = 0;
    for(int i = 0; i < s.length(); i++) {
      char c = s.charAt(i);
      if (i%2==0) {
        if (c == '0') {
          ans1++;
        } else {
          ans2++;
        }
      } else {
        if (c == '1') {
          ans1++; 
        } else {
          ans2++;
        }
      }
    }
    System.out.println(Math.min(ans1, ans2));
  }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz