import java.util.Scanner;

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

	String S = sc.next();
	String x[] = S.split("");

	int count = 0;
	if (x.length < 2)
	    System.out.println(0);
	else {
	    for (int i = 1; i < x.length; i++) {
		if (x[i - 1].equals(x[i])) {
		    if (x[i].equals("0"))
			x[i] = "1";
		    else if (x[i].equals("1"))
			x[i] = "0";
		    count++;
		}
	    }
	    System.out.println(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