import java.util.*;

public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        int a=0;
        int b=0;
        for (int i=0;i<s.length();i=i+2){
            int cur = s.charAt(i)-'0';
            if (cur == 1 ) ++a;
            else ++b;
        }
        for (int i=1;i<s.length();i=i+2){
            int cur = s.charAt(i)-'0';
            if (cur == 0 ) ++a;
            else ++b;
        }
        System.out.println(Math.min(a,b));
    }
}

/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