import java.io.BufferedReader;
import java.io.*;
import java.util.*;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Collections;
import java.util.Arrays;

public class Main {
	public static void main(final String[] args){
    
    final Scanner sc = new Scanner(System.in);
    final List<Integer> list = new ArrayList<>();
    //final List<Long> list2 = new ArrayList<>();

    String s = sc.next();

    char[] work = new char[s.length()];
    int count = 0;

    for(int i = 0; i < s.length(); i++){
            work[i] = s.charAt(i);
    }
    for(int i = 0; i < s.length() - 1; i++){
        if(work[i] == work[i+1]){
            if(work[i+1] == '1'){
                work[i+1] = '0';
            }else{
                work[i+1] = '1';
            }
            count++;
        }
    }
    System.out.println(count);




}
} import java.io.BufferedReader; BufferedReader java.io import java.io.*; io.* java import java.util.*; util.* java import java.util.ArrayList; ArrayList java.util import java.util.Iterator; Iterator java.util import java.util.List; List java.util import java.util.Collections; Collections java.util import java.util.Arrays; Arrays java.util public class Main {
	public static void main(final String[] args){
    
    final Scanner sc = new Scanner(System.in);
    final List<Integer> list = new ArrayList<>();
    //final List<Long> list2 = new ArrayList<>();

    String s = sc.next();

    char[] work = new char[s.length()];
    int count = 0;

    for(int i = 0; i < s.length(); i++){
            work[i] = s.charAt(i);
    }
    for(int i = 0; i < s.length() - 1; i++){
        if(work[i] == work[i+1]){
            if(work[i+1] == '1'){
                work[i+1] = '0';
            }else{
                work[i+1] = '1';
            }
            count++;
        }
    }
    System.out.println(count);




}
} Main public static void main(final String[] args){
    
    final Scanner sc = new Scanner(System.in);
    final List<Integer> list = new ArrayList<>();
    //final List<Long> list2 = new ArrayList<>();

    String s = sc.next();

    char[] work = new char[s.length()];
    int count = 0;

    for(int i = 0; i < s.length(); i++){
            work[i] = s.charAt(i);
    }
    for(int i = 0; i < s.length() - 1; i++){
        if(work[i] == work[i+1]){
            if(work[i+1] == '1'){
                work[i+1] = '0';
            }else{
                work[i+1] = '1';
            }
            count++;
        }
    }
    System.out.println(count);




} main {
    
    final Scanner sc = new Scanner(System.in);
    final List<Integer> list = new ArrayList<>();
    //final List<Long> list2 = new ArrayList<>();

    String s = sc.next();

    char[] work = new char[s.length()];
    int count = 0;

    for(int i = 0; i < s.length(); i++){
            work[i] = s.charAt(i);
    }
    for(int i = 0; i < s.length() - 1; i++){
        if(work[i] == work[i+1]){
            if(work[i+1] == '1'){
                work[i+1] = '0';
            }else{
                work[i+1] = '1';
            }
            count++;
        }
    }
    System.out.println(count);




} final Scanner sc = new Scanner(System.in); sc new Scanner(System.in) final List<Integer> list = new ArrayList<>(); list new ArrayList<>() String s = sc.next(); s sc.next() sc.next sc char[] work = new char[s.length()]; work new char[s.length()] s.length() s.length s int count = 0; count 0 for(int i = 0; i < s.length(); i++){
            work[i] = s.charAt(i);
    } int i = 0; int i = 0; i 0 i < s.length() i s.length() s.length s i++ i++ i {
            work[i] = s.charAt(i);
    } {
            work[i] = s.charAt(i);
    } work[i] = s.charAt(i) work[i] work i s.charAt(i) s.charAt s i for(int i = 0; i < s.length() - 1; i++){
        if(work[i] == work[i+1]){
            if(work[i+1] == '1'){
                work[i+1] = '0';
            }else{
                work[i+1] = '1';
            }
            count++;
        }
    } int i = 0; int i = 0; i 0 i < s.length() - 1 i s.length() - 1 s.length() s.length s 1 i++ i++ i {
        if(work[i] == work[i+1]){
            if(work[i+1] == '1'){
                work[i+1] = '0';
            }else{
                work[i+1] = '1';
            }
            count++;
        }
    } {
        if(work[i] == work[i+1]){
            if(work[i+1] == '1'){
                work[i+1] = '0';
            }else{
                work[i+1] = '1';
            }
            count++;
        }
    } if(work[i] == work[i+1]){
            if(work[i+1] == '1'){
                work[i+1] = '0';
            }else{
                work[i+1] = '1';
            }
            count++;
        } work[i] == work[i+1] work[i] work i work[i+1] work i+1 i 1 {
            if(work[i+1] == '1'){
                work[i+1] = '0';
            }else{
                work[i+1] = '1';
            }
            count++;
        } if(work[i+1] == '1'){
                work[i+1] = '0';
            }else{
                work[i+1] = '1';
            } work[i+1] == '1' work[i+1] work i+1 i 1 '1' {
                work[i+1] = '0';
            } work[i+1] = '0' work[i+1] work i+1 i 1 '0' {
                work[i+1] = '1';
            } work[i+1] = '1' work[i+1] work i+1 i 1 '1' count++ count System.out.println(count) System.out.println System.out System System.out count final String[] args args public class Main {
	public static void main(final String[] args){
    
    final Scanner sc = new Scanner(System.in);
    final List<Integer> list = new ArrayList<>();
    //final List<Long> list2 = new ArrayList<>();

    String s = sc.next();

    char[] work = new char[s.length()];
    int count = 0;

    for(int i = 0; i < s.length(); i++){
            work[i] = s.charAt(i);
    }
    for(int i = 0; i < s.length() - 1; i++){
        if(work[i] == work[i+1]){
            if(work[i+1] == '1'){
                work[i+1] = '0';
            }else{
                work[i+1] = '1';
            }
            count++;
        }
    }
    System.out.println(count);




}
} public class Main {
	public static void main(final String[] args){
    
    final Scanner sc = new Scanner(System.in);
    final List<Integer> list = new ArrayList<>();
    //final List<Long> list2 = new ArrayList<>();

    String s = sc.next();

    char[] work = new char[s.length()];
    int count = 0;

    for(int i = 0; i < s.length(); i++){
            work[i] = s.charAt(i);
    }
    for(int i = 0; i < s.length() - 1; i++){
        if(work[i] == work[i+1]){
            if(work[i+1] == '1'){
                work[i+1] = '0';
            }else{
                work[i+1] = '1';
            }
            count++;
        }
    }
    System.out.println(count);




}
} Main
import java.io.BufferedReader;
import java.io.*;
import java.util.*;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Collections;
import java.util.Arrays;

public class Main {
	public static void main(final String[] args){
    
    final Scanner sc = new Scanner(System.in);
    final List<Integer> list = new ArrayList<>();
    //final List<Long> list2 = new ArrayList<>();

    String s = sc.next();

    char[] work = new char[s.length()];
    int count = 0;

    for(int i = 0; i < s.length(); i++){
            work[i] = s.charAt(i);
    }
    for(int i = 0; i < s.length() - 1; i++){
        if(work[i] == work[i+1]){
            if(work[i+1] == '1'){
                work[i+1] = '0';
            }else{
                work[i+1] = '1';
            }
            count++;
        }
    }
    System.out.println(count);




}
}