import java.util.*;

public class Main{

    public static void main(String args[]){
        Scanner sc = new Scanner(System.in);
        String S = sc.next();
        sc.close();
        int count1 = 0;
        int count2 = 0;
        
        for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '1')
                    count1++;
            } else{
                if(S.charAt(i) == '0')
                    count1++;
            }
        }

        for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '0')
                    count2++;
            } else{
                if(S.charAt(i) == '1')
                    count2++;
            }
        }
        System.out.println(Math.min(count1, count2));
    }
}

/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