import java.util.*; 

class Main{
public static void main(String[] args){
	Scanner sc=new Scanner(System.in);
        String s=sc.next();
   		String[] a=s.split("");
  		int ans=0;

		for(int c=0;c<a.length-1;c++){
    		if(a[c].equals("0")&&a[c+1].equals("0")){
              a[c+1]="1";
              ans++;
            }else if(a[c].equals("1")&&a[c+1].equals("1")){
              a[c+1]="0";
              ans++;
            }
    	}
  
    	System.out.println(ans);
    }
}

/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