import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);
		String s = sc.next();

		int count01 = 0;
		int count10 = 0;

		for (int i = 0; i < s.length(); i++) {
			if (Integer.parseInt(s.substring(i, i + 1)) == i % 2) {
				count01++;
			}
		}
		count10 = s.length() - count01;
		System.out.println(Math.min(count01, count10));

		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