import java.util.*;

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

		int count_1 = 0;
		for (int i=0; i<S.length(); i++){
			if (i%2 == 0){
				if (S.charAt(i) == '0'){
					count_1 += 1;
				}
			}else if (i%2 == 1){
				if (S.charAt(i) == '1'){
					count_1 += 1;
				}
			}
		}

		int count_2 = 0;
		for (int i=0; i<S.length(); i++){
			if (i%2 == 0){
				if (S.charAt(i) == '1'){
					count_2 += 1;
				}
			}else if (i%2 == 1){
				if (S.charAt(i) == '0'){
					count_2 += 1;
				}
			}
		}

		System.out.println(Math.min(count_1, count_2));
	}
}
/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