import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        String str = scan.next();
        int min1=0;
        int min2=0;
        for (int i=0 ; i<str.length();i++){
        	if(i%2 ==Character.getNumericValue(str.charAt(i))){
        		min1 +=1;
        	}else{
        		min2 +=1;
        	}
        }
        System.out.println(Math.min(min1,min2));
    }
}
/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