import java.util.*;
class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String S[] = sc.next().split("");
    String a = "1";
    String b = "0";
    int countA = 0;
    int countB = 0;
    for (String s : S) {
      if (s.equals(a)) {
        countA++;	
      }
      if (s.equals(b)) {
        countB++;
      }
      String t = a;
      a = b;
      b = t;
    }
    System.out.println(Math.min(countA,countB));
  }
}
/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