
import java.util.Scanner;

public class Main {
    public static void main(final String[] args) {
        final Scanner scanner = new Scanner(System.in);
        final char[] chars = scanner.nextLine().toCharArray();

        int resultA = 0;
        int resultB = 0;

        for (int i = 0; i < chars.length; i++) {
            if (i % 2 == 0) {
                if (chars[i] == '0') {
                    resultA++;
                } else {
                    resultB++;
                }
            } else {
                if (chars[i] == '1') {
                    resultA++;
                } else {
                    resultB++;
                }
            }
        }

        System.out.println(Math.min(resultA, resultB));
    }
}

/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