import java.util.Scanner;

public class Main {
    public static void main(String[] args) {

        Scanner s = new Scanner(System.in);
        char[] chars = s.next().toCharArray();
        int[] n = new int[chars.length];

        for (int i = 0; i < chars.length; i++) {
            n[i] = Character.getNumericValue(chars[i]);
        }

        int wb = 0;
        int bw = 0;

        for (int i = 0; i < chars.length; i++) {
            if (i % 2 == 1 && n[i] == 0) {
                wb++;
            } else if (i % 2 == 0 && n[i] == 1) {
                wb++;
            } else if (i % 2 == 1 && n[i] == 1) {
                bw++;
            } else if (i % 2 == 0 && n[i] == 0) {
                bw++;
            }
        }
        System.out.println(wb < bw ? wb : bw);
    }
}
/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