import java.util.*;

public class Main{

    public static void main(String args[]){
        Scanner sc = new Scanner(System.in);
        String S = sc.next();
        sc.close();
        int count1 = 0;
        int count2 = 0;
        
        for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '1')
                    count1++;
            } else{
                if(S.charAt(i) == '0')
                    count1++;
            }
        }

        for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '0')
                    count2++;
            } else{
                if(S.charAt(i) == '1')
                    count2++;
            }
        }
        System.out.println(Math.min(count1, count2));
    }
} import java.util.*; util.* java public class Main{

    public static void main(String args[]){
        Scanner sc = new Scanner(System.in);
        String S = sc.next();
        sc.close();
        int count1 = 0;
        int count2 = 0;
        
        for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '1')
                    count1++;
            } else{
                if(S.charAt(i) == '0')
                    count1++;
            }
        }

        for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '0')
                    count2++;
            } else{
                if(S.charAt(i) == '1')
                    count2++;
            }
        }
        System.out.println(Math.min(count1, count2));
    }
} Main public static void main(String args[]){
        Scanner sc = new Scanner(System.in);
        String S = sc.next();
        sc.close();
        int count1 = 0;
        int count2 = 0;
        
        for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '1')
                    count1++;
            } else{
                if(S.charAt(i) == '0')
                    count1++;
            }
        }

        for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '0')
                    count2++;
            } else{
                if(S.charAt(i) == '1')
                    count2++;
            }
        }
        System.out.println(Math.min(count1, count2));
    } main {
        Scanner sc = new Scanner(System.in);
        String S = sc.next();
        sc.close();
        int count1 = 0;
        int count2 = 0;
        
        for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '1')
                    count1++;
            } else{
                if(S.charAt(i) == '0')
                    count1++;
            }
        }

        for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '0')
                    count2++;
            } else{
                if(S.charAt(i) == '1')
                    count2++;
            }
        }
        System.out.println(Math.min(count1, count2));
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) String S = sc.next(); S sc.next() sc.next sc sc.close() sc.close sc int count1 = 0; count1 0 int count2 = 0; count2 0 for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '1')
                    count1++;
            } else{
                if(S.charAt(i) == '0')
                    count1++;
            }
        } int i = 0; int i = 0; i 0 i < S.length() i S.length() S.length S i++ i++ i {
            if(i%2 == 0) {
                if(S.charAt(i) == '1')
                    count1++;
            } else{
                if(S.charAt(i) == '0')
                    count1++;
            }
        } {
            if(i%2 == 0) {
                if(S.charAt(i) == '1')
                    count1++;
            } else{
                if(S.charAt(i) == '0')
                    count1++;
            }
        } if(i%2 == 0) {
                if(S.charAt(i) == '1')
                    count1++;
            } else{
                if(S.charAt(i) == '0')
                    count1++;
            } i%2 == 0 i%2 i 2 0 {
                if(S.charAt(i) == '1')
                    count1++;
            } if(S.charAt(i) == '1')
                    count1++; S.charAt(i) == '1' S.charAt(i) S.charAt S i '1' count1++ count1 {
                if(S.charAt(i) == '0')
                    count1++;
            } if(S.charAt(i) == '0')
                    count1++; S.charAt(i) == '0' S.charAt(i) S.charAt S i '0' count1++ count1 for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '0')
                    count2++;
            } else{
                if(S.charAt(i) == '1')
                    count2++;
            }
        } int i = 0; int i = 0; i 0 i < S.length() i S.length() S.length S i++ i++ i {
            if(i%2 == 0) {
                if(S.charAt(i) == '0')
                    count2++;
            } else{
                if(S.charAt(i) == '1')
                    count2++;
            }
        } {
            if(i%2 == 0) {
                if(S.charAt(i) == '0')
                    count2++;
            } else{
                if(S.charAt(i) == '1')
                    count2++;
            }
        } if(i%2 == 0) {
                if(S.charAt(i) == '0')
                    count2++;
            } else{
                if(S.charAt(i) == '1')
                    count2++;
            } i%2 == 0 i%2 i 2 0 {
                if(S.charAt(i) == '0')
                    count2++;
            } if(S.charAt(i) == '0')
                    count2++; S.charAt(i) == '0' S.charAt(i) S.charAt S i '0' count2++ count2 {
                if(S.charAt(i) == '1')
                    count2++;
            } if(S.charAt(i) == '1')
                    count2++; S.charAt(i) == '1' S.charAt(i) S.charAt S i '1' count2++ count2 System.out.println(Math.min(count1, count2)) System.out.println System.out System System.out Math.min(count1, count2) Math.min Math count1 count2 String args[] args public class Main{

    public static void main(String args[]){
        Scanner sc = new Scanner(System.in);
        String S = sc.next();
        sc.close();
        int count1 = 0;
        int count2 = 0;
        
        for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '1')
                    count1++;
            } else{
                if(S.charAt(i) == '0')
                    count1++;
            }
        }

        for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '0')
                    count2++;
            } else{
                if(S.charAt(i) == '1')
                    count2++;
            }
        }
        System.out.println(Math.min(count1, count2));
    }
} public class Main{

    public static void main(String args[]){
        Scanner sc = new Scanner(System.in);
        String S = sc.next();
        sc.close();
        int count1 = 0;
        int count2 = 0;
        
        for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '1')
                    count1++;
            } else{
                if(S.charAt(i) == '0')
                    count1++;
            }
        }

        for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '0')
                    count2++;
            } else{
                if(S.charAt(i) == '1')
                    count2++;
            }
        }
        System.out.println(Math.min(count1, count2));
    }
} Main
import java.util.*;

public class Main{

    public static void main(String args[]){
        Scanner sc = new Scanner(System.in);
        String S = sc.next();
        sc.close();
        int count1 = 0;
        int count2 = 0;
        
        for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '1')
                    count1++;
            } else{
                if(S.charAt(i) == '0')
                    count1++;
            }
        }

        for(int i = 0; i < S.length(); i++){
            if(i%2 == 0) {
                if(S.charAt(i) == '0')
                    count2++;
            } else{
                if(S.charAt(i) == '1')
                    count2++;
            }
        }
        System.out.println(Math.min(count1, count2));
    }
}
