import java.util.Scanner;

public class Main
{

	public static void main(String[] args)
	{
		Scanner sc = new Scanner(System.in);

		String s = sc.next();
		//int num = Integer.parseInt(s, 2);

		int whitefirst = 0;

		for (int i = 0; i < s.length(); i++) {
			if (s.charAt(i) - '0' == i % 2) {
				whitefirst++;
			}
		}

		int blackfirst = 0;

		for (int i = 0; i < s.length(); i++) {
			if (s.charAt(i) - '0' != i % 2) {
				blackfirst++;
			}
		}

		System.out.println(Math.min(whitefirst, blackfirst));
		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