import java.util.*;
public class Main{
	public static void main(String[] args){
      Scanner sc = new Scanner(System.in);
      String s = sc.next();
      int n = s.length();
      int cnt1 = 0;
      int cnt2 = 0;
      for(int i = 0;i < n;i+=2){
      	if(s.substring(i,i+1).equals("0"))cnt1++;
      }
      for(int i = 1;i < n;i+=2){
      	if(s.substring(i,i+1).equals("1"))cnt1++;
      }
      for(int i = 0;i < n;i+=2){
      	if(s.substring(i,i+1).equals("1"))cnt2++;
      }
      for(int i = 1;i < n;i+=2){
      	if(s.substring(i,i+1).equals("0"))cnt2++;
      }
      System.out.println(Math.min(cnt1,cnt2));
    }
}
/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