import java.util.*;
public class Main { 
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.next();
        char[] chs = str.toCharArray();
        int evenCnt = 0; // even digit is 0
        int oddCnt = 0; // odd digit is 0

        for(int i = 0; i < chs.length; i++) {
          if(i % 2 == 0) {// even digit
            if(chs[i] == '1')
              evenCnt++;
            else
              oddCnt++;
          } else { // odd digit
            if(chs[i] == '0')
              evenCnt++;
            else
              oddCnt++;
          }
        }
        System.out.println(Math.min(evenCnt, oddCnt));
    }
}
/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