import java.util.Scanner;

class Main {
	public static void main(String[] $) {
		Scanner sc = new Scanner(System.in);
		String s=sc.next(),t;
		int countOdd=0,countEven=0;
		for(int i=0;i<s.length();i++) {
			if(i%2==0)t="0";
			else t="1";
			if(!s.substring(i, i+1).equals(t))countOdd++;
		}
		for(int i=0;i<s.length();i++) {
			if(i%2==0)t="1";
			else t="0";
			if(!s.substring(i, i+1).equals(t))countEven++;
		}
		System.out.println((int)Math.min(countOdd,countEven));
	}
}


/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