import java.util.Scanner;

public class Main {
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String S = sc.nextLine();
		
		int k = 0;
		int g = 0;
		for (int i = 0; i < S.length(); i++) {
			if (S.charAt(i) == '0') {
				if (i % 2 == 0) {
					g++;
				} else {
					k++;
				}
			} else {
				if (i % 2 == 0) {
					k++;
				} else {
					g++;
				}
			}
		}
		
		System.out.println(Math.min(k, g));
	}
}

/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