import java.util.Scanner;
 
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String input = sc.nextLine();
		int countA = 0;
		int countB = 0;
		int A =0;
		int B =1;
		for (int i = 0; i < input.length() ; i++) {
			int tmp = combertInt(input,i);
			if(A == tmp) {
            	countA++;
            }
          	if(A==0) {
				A=1;
			}else{
				A=0;
			}
          	if(B == tmp) {
            	countB++;
            }
			if(B==1) {
				B=0;
			}else{
				B=1;
			}
		}
		System.out.println(Math.min(countA, countB));
	}
	
	private static Integer combertInt(String s, int i) {
		return Integer.parseInt(s.substring(i,i+1));
	}
}
/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