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