import java.util.Scanner;

public class Main {
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		String tile = sc.nextLine();
		sc.close();
		int ans = tile.length();

		for (int i = 0; i < 2; i++) {
			int cnt = 0;

			for (int j = 0; j < tile.length(); j++) {
				int current = Integer.parseInt(String.valueOf(tile.charAt(j)));
				if (j % 2 == 0 && tile.charAt(j) != (char) (i + '0')) {
					cnt++;
				}
				if (j % 2 == 1 && tile.charAt(j) == (char) (i + '0')) {
					cnt++;
				}
			}
			ans = Math.min(ans, cnt);

		}
		System.out.println(ans);

	}

}
/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