import java.util.*;
public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String S = sc.next();
    
    int count01 = 0;
    int count10 = 0;
    for (int i = 0; i < S.length(); i++) {
      char c = S.charAt(i);
      if (i%2 == 0) {
        if (c == '0') {
          count10++;
        } else {
          count01++;
        }
      } else {
        if (c == '1') {
          count10++;
        } else {
          count01++;
        }
      }
    }
        
    System.out.println(Math.min(count01, count10));
  }  
}
/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