
import java.util.*;
import java.math.*;
public class Main {
	
	public static void main(String[] args){
		
		Scanner sc = new Scanner(System.in);
		int dif = 0;
		String N = sc.next();
		
		for (int i = 0; i < N.length();i+=2)
		{
			char c1 = N.charAt(i);
			if (c1 == '1') {dif++;}
			
			if(i+1 < N.length())
			{
			char c2 = N.charAt(i+1);	
			if (c2 == '0') {dif++;}
			}
			
		}
      
      int ans = Math.min(dif,(N.length()-dif));
		
		System.out.println(ans);
		
	

	}
}
/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