import java.util.*;

public class Main{

	public static void main(String[] args){
		Scanner sc =new Scanner(System.in);
		String tile = sc.next();
		//System.out.println("tile");
		String[] tilec = tile.split("");

		int counter1=0;
		int counter2=0;
		for(int i=0;i<tilec.length;i++){
			if(i%2 == 0){
				if(tilec[i].equals("0")){
					counter1++;
				}
			}else{
				if(tilec[i].equals("1")){
					counter1++;
				}
			}
		}

		for(int i=0;i<tilec.length;i++){
			if(i%2 == 0){
				if(tilec[i].equals("1") ){
					counter2++;
				}
			}else{
				if(tilec[i].equals("0") ){
					counter2++;
				}
			}
		}

		if(counter1 >= counter2){
			System.out.println(counter2);
		}else{
			System.out.println(counter1);
		}
	}
}
/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