import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);

    char[] s = scanner.next().toCharArray();
    int min = s.length;
    for (int i = 0; i < 2; i++) {
      int cnt = 0;
      for (int j = 0; j < s.length; j++) {
        if (s[j] - '0' + Math.abs(i - j % 2) == 1) cnt++;
      }
      min = Math.min(min, cnt);
    }
    System.out.println(min);
  }
}

/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