import java.util.Scanner;

public class Main {

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

		String s = sc.next();
		String [] str = s.split("");
		int[] n = new int[s.length()];
		for (int i = 0; i < s.length(); i++) {
			n[i] = Integer.parseInt(str[i]);
		}
		int cnt = 0;
		for (int i = 0; i < s.length() - 1; i++) {
			if (n[i] == n[i+1]) {
				cnt++;
				if(n[i+1] == 0){
					n[i+1] = 1;
				}else{
					n[i+1] = 0;
				}
			}
		}
		System.out.println(cnt);
	}
}
/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