import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        boolean[] bs = new boolean[s.length()];
        boolean[] p = new boolean[s.length()];
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '0') {
                bs[i] = false;
            } else {
                bs[i] = true;
            }
            if (i % 2 == 0) {
                p[i] = false;
            } else {
                p[i] = true;
            }
        }
        int counter1 = 0;
        int counter2 = 0;
        for (int i = 0; i < bs.length; i++) {
            if (bs[i] != p[i]) {
                counter1++;
            } else {
                counter2++;
            }
        }
        System.out.println(Math.min(counter1, counter2));
        sc.close();
    }
}
/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