import java.util.Scanner;

class Main{

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String s = sc.nextLine();
		int count = 0;
		String t;
		
		for(int i = 1; i <= s.length(); i++) {
			t = s.substring(i-1,i);
			if(t.equals(String.valueOf(i % 2))) {
				count ++;
			}
		}
			System.out.println(Math.min(count, s.length() - count));
	}
}
/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