import java.util.Scanner;
public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		String tile = sc.next();
		int cnt0 = 0;
		int cnt1 = 0;
		for(int i = 0; i < tile.length(); i++){
			if(i%2 == 0){
				if(tile.charAt(i) != '0'){
					cnt0++;
				}
			}
			if(i%2 == 1){
				if(tile.charAt(i) != '1'){
					cnt0++;
				}
			}
			if(i%2 == 0){
				if(tile.charAt(i) != '1'){
					cnt1++;
				}
			}
			if(i%2 == 1){
				if(tile.charAt(i) != '0'){
					cnt1++;
				}
			}
		}
		System.out.println(Math.min(cnt0,cnt1));
	}
}
/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