

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);

		char[] str = sc.next().toCharArray();

		int count = 0;
		int tmpCount = 0;

		for(int i = 0; i < str.length; i++){
			if((i % 2 == 0 && str[i] == '0') ||( i % 2 == 1 && str[i] == '1')){
				count++;
			}
		}

		for(int i = 0; i < str.length; i++){
			if((i % 2 == 1 && str[i] == '0') ||( i % 2 == 0 && str[i] == '1')){
				tmpCount++;
			}
		}

		System.out.println(Math.min(count, tmpCount));

	}

}
/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