import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String s = sc.nextLine();
		char c[] = s.toCharArray();
		int couA=0;
int couB=0;
		for (int i = 0; i < c.length; i++) {
			if (i % 2 == 0) {
				if(c[i]=='0'){couA++;}
			} else {
				if(c[i]!='0'){couA++;}
			}
		}
		for (int i = 0; i < c.length; i++) {
			if (i % 2 != 0) {
				if(c[i]=='0'){couB++;}
			} else {
				if(c[i]!='0'){couB++;}
			}
		}
	System.out.println(Math.min(couA, couB));
	}
}
/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