import java.util.*;
class Main{
  public static void main(String args[]){
    Scanner s=new Scanner(System.in);
    String S=s.next();
    int r=0;
    char b=S.charAt(0);
    for(int i=1;i<S.length();i++){
      if(S.charAt(i)==b){
        r++;
        b=(S.charAt(i)=='0'?'1':'0');
      }
      else b=S.charAt(i);
    }
    System.out.println(r);
  }
}

/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