import java.io.*;
public class Main{
    public static void main(String[] args) throws IOException{
        BufferedReader bf = 
            new BufferedReader(new InputStreamReader(System.in));
        String[] str = bf.readLine().split("");
        int num1 = 0,
            num2 = 0;
        for(int i = 0; i < str.length; i++){
            int color = Integer.parseInt(str[i]);
            if(i%2 == color){
                num1++;
            }
            else{
                num2++;
            }
        }
        System.out.println(Math.min(num1, num2));
    }
}
/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