import java.util.*;

public class Main{

    public static List<Integer> P;
    public static void main(String args[]){
        Scanner scanner = new Scanner(System.in);
        String S = scanner.next();
        char [] cha = S.toCharArray();

        int countFrom1 = 0;
        int countFrom0 = 0;
        for(int i=0; i<S.length(); i++){
            if(i%2==0 && cha[i]=='1'){
                countFrom0++;
            }
            if(i%2==1 && cha[i]=='0'){
                countFrom0++;
            }
            if(i%2==0 && cha[i]=='0'){
                countFrom1++;
            }
            if(i%2==1 && cha[i]=='1'){
                countFrom1++;
            }
        }
        System.out.println(Math.min(countFrom0, countFrom1));
    }
}
/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