import java.util.*;

public class Main{
    public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);

      String s = sc.next();
      int lng = s.length();
      char[] c = new char[lng];
      for(int i=0;i<lng;i++){
        c[i] = s.charAt(i);
      }

      int ans1 = 0;
      int ans2 = 0;

      for(int i=0;i<lng;i++){
        if(i%2==0 && c[i]=='1'){
          ans1++;
        }
        if(i%2==1 && c[i]=='0'){
          ans1++;
        }
      }

      for(int i=0;i<lng;i++){
        if((i+1)%2==1 && c[i]=='0'){
          ans2++;
        }
        if((i+1)%2==0 && c[i]=='1'){
          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