import java.util.*;

public class Main {
    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        int n = s.length();
        boolean is0 = true;
        int count = 0;
        for (char c : s.toCharArray()) {
            if (is0 && c == '0') count++;
            if (!is0 && c != '0') count++;
            is0 = !is0;
        }
        System.out.println(Math.min(count, n - 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