import java.util.Scanner;

public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int ans;
    char[] nums = sc.nextLine().toCharArray();
    int numLength = nums.length;
    int bw = 0;
    int wb = 0;
    for(int cnt = 0; cnt < numLength ; cnt++){
      if( nums[cnt] == ((cnt % 2 == 0)? '0': '1' )){
        bw++;
      }
      else {
        wb++; 
      }
    }
    ans = Math.min(bw,wb);
    System.out.println(ans);
    sc.close();
  }
}
/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