import java.util.*;

public class Main{
  public static void main(String[]args){
    Scanner sc = new Scanner(System.in);
    String S = sc.nextLine();
    int n1 =0, n2 = 0;
    for(int i = 0; i < S.length(); i++){
      if((i%2 == 0 && S.charAt(i) == '0') || (i%2 == 1 && S.charAt(i) == '1')){
        n1++;
      }else{
        n2++;
      }
    }
    System.out.println(Math.min(n1, n2));
  }
}
/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