import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String s = sc.next();
		int ans0 = 0;
		int ans1 = 0;
		for (int i = 0; i < s.length(); i++) {
			int target = s.charAt(i)-'0';
			if (i % 2 == 0) {
				if (0 == target) {ans0++;}
				if (1 == target) {ans1++;}
			}else {
				if (0 ==  target) {ans1++;}
				if (1 ==  target) {ans0++;}
			}
		}
		System.out.println(Math.min(ans0, ans1));
	}

}



/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