import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String s = scanner.next();

        int[] counts = new int[2];
        for (int i = 0; i < s.length(); i++) {
            int c = s.charAt(i) - '0';
            counts[(i + c) % 2]++;
        }

        System.out.println(Math.min(counts[0], counts[1]));
    }
}

/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