
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class Main {
    
	public static void main(String[] args) {
	  FastReader sc = new FastReader();
	  String s = sc.next();
	  int n = s.length();
	  // There are only two pattern s will transform to...
	  // 101010101010101010...
	  // 010101010101010101...
	  char p1 = '0';
	  char p2 = '1';
	  int pattern1 = 0;
	  int pattern2 = 0;
	  
	  for(char c: s.toCharArray()) {
		  if (c != p1) { pattern1++; }
		  if (c != p2) { pattern2++; }
		  
		  p1 = (p1 == '0') ? '1' : '0';
		  p2 = (p2 == '1') ? '0' : '1';
	  }
	  
	  System.out.println(Math.min(pattern1, pattern2));
	  
  }

  static class FastReader {
    BufferedReader br;
    StringTokenizer st;

    public FastReader() {
      br = new BufferedReader(new InputStreamReader(System.in));
    }
  
    String next() { 
      while (st == null || !st.hasMoreElements()) {
        try {
          st = new StringTokenizer(br.readLine());
        } catch (IOException  e) {
          e.printStackTrace();
        }
      }
      return st.nextToken();
    }

    int nextInt() {
      return Integer.parseInt(next());
    }

    long nextLong() {
      return Long.parseLong(next());
    }

    double nextDouble() { 
      return Double.parseDouble(next());
    }

    String nextLine() {
      String str = "";
        try{
            str = br.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return str;
      }
  }
} import java.io.BufferedReader; BufferedReader java.io import java.io.IOException; IOException java.io import java.io.InputStreamReader; InputStreamReader java.io import java.util.StringTokenizer; StringTokenizer java.util public class Main {
    
	public static void main(String[] args) {
	  FastReader sc = new FastReader();
	  String s = sc.next();
	  int n = s.length();
	  // There are only two pattern s will transform to...
	  // 101010101010101010...
	  // 010101010101010101...
	  char p1 = '0';
	  char p2 = '1';
	  int pattern1 = 0;
	  int pattern2 = 0;
	  
	  for(char c: s.toCharArray()) {
		  if (c != p1) { pattern1++; }
		  if (c != p2) { pattern2++; }
		  
		  p1 = (p1 == '0') ? '1' : '0';
		  p2 = (p2 == '1') ? '0' : '1';
	  }
	  
	  System.out.println(Math.min(pattern1, pattern2));
	  
  }

  static class FastReader {
    BufferedReader br;
    StringTokenizer st;

    public FastReader() {
      br = new BufferedReader(new InputStreamReader(System.in));
    }
  
    String next() { 
      while (st == null || !st.hasMoreElements()) {
        try {
          st = new StringTokenizer(br.readLine());
        } catch (IOException  e) {
          e.printStackTrace();
        }
      }
      return st.nextToken();
    }

    int nextInt() {
      return Integer.parseInt(next());
    }

    long nextLong() {
      return Long.parseLong(next());
    }

    double nextDouble() { 
      return Double.parseDouble(next());
    }

    String nextLine() {
      String str = "";
        try{
            str = br.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return str;
      }
  }
} Main public static void main(String[] args) {
	  FastReader sc = new FastReader();
	  String s = sc.next();
	  int n = s.length();
	  // There are only two pattern s will transform to...
	  // 101010101010101010...
	  // 010101010101010101...
	  char p1 = '0';
	  char p2 = '1';
	  int pattern1 = 0;
	  int pattern2 = 0;
	  
	  for(char c: s.toCharArray()) {
		  if (c != p1) { pattern1++; }
		  if (c != p2) { pattern2++; }
		  
		  p1 = (p1 == '0') ? '1' : '0';
		  p2 = (p2 == '1') ? '0' : '1';
	  }
	  
	  System.out.println(Math.min(pattern1, pattern2));
	  
  } main {
	  FastReader sc = new FastReader();
	  String s = sc.next();
	  int n = s.length();
	  // There are only two pattern s will transform to...
	  // 101010101010101010...
	  // 010101010101010101...
	  char p1 = '0';
	  char p2 = '1';
	  int pattern1 = 0;
	  int pattern2 = 0;
	  
	  for(char c: s.toCharArray()) {
		  if (c != p1) { pattern1++; }
		  if (c != p2) { pattern2++; }
		  
		  p1 = (p1 == '0') ? '1' : '0';
		  p2 = (p2 == '1') ? '0' : '1';
	  }
	  
	  System.out.println(Math.min(pattern1, pattern2));
	  
  } FastReader sc = new FastReader(); sc new FastReader() String s = sc.next(); s sc.next() sc.next sc int n = s.length(); n s.length() s.length s char p1 = '0'; p1 '0' char p2 = '1'; p2 '1' int pattern1 = 0; pattern1 0 int pattern2 = 0; pattern2 0 for(char c: s.toCharArray()) {
		  if (c != p1) { pattern1++; }
		  if (c != p2) { pattern2++; }
		  
		  p1 = (p1 == '0') ? '1' : '0';
		  p2 = (p2 == '1') ? '0' : '1';
	  } char c s.toCharArray() s.toCharArray s {
		  if (c != p1) { pattern1++; }
		  if (c != p2) { pattern2++; }
		  
		  p1 = (p1 == '0') ? '1' : '0';
		  p2 = (p2 == '1') ? '0' : '1';
	  } {
		  if (c != p1) { pattern1++; }
		  if (c != p2) { pattern2++; }
		  
		  p1 = (p1 == '0') ? '1' : '0';
		  p2 = (p2 == '1') ? '0' : '1';
	  } if (c != p1) { pattern1++; } c != p1 c p1 { pattern1++; } pattern1++ pattern1 if (c != p2) { pattern2++; } c != p2 c p2 { pattern2++; } pattern2++ pattern2 p1 = (p1 == '0') ? '1' : '0' p1 (p1 == '0') ? '1' : '0' (p1 == '0') p1 '0' '1' '0' p2 = (p2 == '1') ? '0' : '1' p2 (p2 == '1') ? '0' : '1' (p2 == '1') p2 '1' '0' '1' System.out.println(Math.min(pattern1, pattern2)) System.out.println System.out System System.out Math.min(pattern1, pattern2) Math.min Math pattern1 pattern2 String[] args args static class FastReader {
    BufferedReader br;
    StringTokenizer st;

    public FastReader() {
      br = new BufferedReader(new InputStreamReader(System.in));
    }
  
    String next() { 
      while (st == null || !st.hasMoreElements()) {
        try {
          st = new StringTokenizer(br.readLine());
        } catch (IOException  e) {
          e.printStackTrace();
        }
      }
      return st.nextToken();
    }

    int nextInt() {
      return Integer.parseInt(next());
    }

    long nextLong() {
      return Long.parseLong(next());
    }

    double nextDouble() { 
      return Double.parseDouble(next());
    }

    String nextLine() {
      String str = "";
        try{
            str = br.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return str;
      }
  } FastReader BufferedReader br; br StringTokenizer st; st public FastReader() {
      br = new BufferedReader(new InputStreamReader(System.in));
    } FastReader {
      br = new BufferedReader(new InputStreamReader(System.in));
    } br = new BufferedReader(new InputStreamReader(System.in)) br new BufferedReader(new InputStreamReader(System.in)) String next() { 
      while (st == null || !st.hasMoreElements()) {
        try {
          st = new StringTokenizer(br.readLine());
        } catch (IOException  e) {
          e.printStackTrace();
        }
      }
      return st.nextToken();
    } next { 
      while (st == null || !st.hasMoreElements()) {
        try {
          st = new StringTokenizer(br.readLine());
        } catch (IOException  e) {
          e.printStackTrace();
        }
      }
      return st.nextToken();
    } while (st == null || !st.hasMoreElements()) {
        try {
          st = new StringTokenizer(br.readLine());
        } catch (IOException  e) {
          e.printStackTrace();
        }
      } st == null || !st.hasMoreElements() st == null st null !st.hasMoreElements() st.hasMoreElements() st.hasMoreElements st {
        try {
          st = new StringTokenizer(br.readLine());
        } catch (IOException  e) {
          e.printStackTrace();
        }
      } try {
          st = new StringTokenizer(br.readLine());
        } catch (IOException  e) {
          e.printStackTrace();
        } catch (IOException  e) {
          e.printStackTrace();
        } IOException  e {
          e.printStackTrace();
        } e.printStackTrace() e.printStackTrace e {
          st = new StringTokenizer(br.readLine());
        } st = new StringTokenizer(br.readLine()) st new StringTokenizer(br.readLine()) return st.nextToken(); st.nextToken() st.nextToken st int nextInt() {
      return Integer.parseInt(next());
    } nextInt {
      return Integer.parseInt(next());
    } return Integer.parseInt(next()); Integer.parseInt(next()) Integer.parseInt Integer next() next long nextLong() {
      return Long.parseLong(next());
    } nextLong {
      return Long.parseLong(next());
    } return Long.parseLong(next()); Long.parseLong(next()) Long.parseLong Long next() next double nextDouble() { 
      return Double.parseDouble(next());
    } nextDouble { 
      return Double.parseDouble(next());
    } return Double.parseDouble(next()); Double.parseDouble(next()) Double.parseDouble Double next() next String nextLine() {
      String str = "";
        try{
            str = br.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return str;
      } nextLine {
      String str = "";
        try{
            str = br.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return str;
      } String str = ""; str "" try{
            str = br.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } IOException e {
            e.printStackTrace();
        } e.printStackTrace() e.printStackTrace e {
            str = br.readLine();
        } str = br.readLine() str br.readLine() br.readLine br return str; str public class Main {
    
	public static void main(String[] args) {
	  FastReader sc = new FastReader();
	  String s = sc.next();
	  int n = s.length();
	  // There are only two pattern s will transform to...
	  // 101010101010101010...
	  // 010101010101010101...
	  char p1 = '0';
	  char p2 = '1';
	  int pattern1 = 0;
	  int pattern2 = 0;
	  
	  for(char c: s.toCharArray()) {
		  if (c != p1) { pattern1++; }
		  if (c != p2) { pattern2++; }
		  
		  p1 = (p1 == '0') ? '1' : '0';
		  p2 = (p2 == '1') ? '0' : '1';
	  }
	  
	  System.out.println(Math.min(pattern1, pattern2));
	  
  }

  static class FastReader {
    BufferedReader br;
    StringTokenizer st;

    public FastReader() {
      br = new BufferedReader(new InputStreamReader(System.in));
    }
  
    String next() { 
      while (st == null || !st.hasMoreElements()) {
        try {
          st = new StringTokenizer(br.readLine());
        } catch (IOException  e) {
          e.printStackTrace();
        }
      }
      return st.nextToken();
    }

    int nextInt() {
      return Integer.parseInt(next());
    }

    long nextLong() {
      return Long.parseLong(next());
    }

    double nextDouble() { 
      return Double.parseDouble(next());
    }

    String nextLine() {
      String str = "";
        try{
            str = br.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return str;
      }
  }
} public class Main {
    
	public static void main(String[] args) {
	  FastReader sc = new FastReader();
	  String s = sc.next();
	  int n = s.length();
	  // There are only two pattern s will transform to...
	  // 101010101010101010...
	  // 010101010101010101...
	  char p1 = '0';
	  char p2 = '1';
	  int pattern1 = 0;
	  int pattern2 = 0;
	  
	  for(char c: s.toCharArray()) {
		  if (c != p1) { pattern1++; }
		  if (c != p2) { pattern2++; }
		  
		  p1 = (p1 == '0') ? '1' : '0';
		  p2 = (p2 == '1') ? '0' : '1';
	  }
	  
	  System.out.println(Math.min(pattern1, pattern2));
	  
  }

  static class FastReader {
    BufferedReader br;
    StringTokenizer st;

    public FastReader() {
      br = new BufferedReader(new InputStreamReader(System.in));
    }
  
    String next() { 
      while (st == null || !st.hasMoreElements()) {
        try {
          st = new StringTokenizer(br.readLine());
        } catch (IOException  e) {
          e.printStackTrace();
        }
      }
      return st.nextToken();
    }

    int nextInt() {
      return Integer.parseInt(next());
    }

    long nextLong() {
      return Long.parseLong(next());
    }

    double nextDouble() { 
      return Double.parseDouble(next());
    }

    String nextLine() {
      String str = "";
        try{
            str = br.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return str;
      }
  }
} Main

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class Main {
    
	public static void main(String[] args) {
	  FastReader sc = new FastReader();
	  String s = sc.next();
	  int n = s.length();
	  // There are only two pattern s will transform to...
	  // 101010101010101010...
	  // 010101010101010101...
	  char p1 = '0';
	  char p2 = '1';
	  int pattern1 = 0;
	  int pattern2 = 0;
	  
	  for(char c: s.toCharArray()) {
		  if (c != p1) { pattern1++; }
		  if (c != p2) { pattern2++; }
		  
		  p1 = (p1 == '0') ? '1' : '0';
		  p2 = (p2 == '1') ? '0' : '1';
	  }
	  
	  System.out.println(Math.min(pattern1, pattern2));
	  
  }

  static class FastReader {
    BufferedReader br;
    StringTokenizer st;

    public FastReader() {
      br = new BufferedReader(new InputStreamReader(System.in));
    }
  
    String next() { 
      while (st == null || !st.hasMoreElements()) {
        try {
          st = new StringTokenizer(br.readLine());
        } catch (IOException  e) {
          e.printStackTrace();
        }
      }
      return st.nextToken();
    }

    int nextInt() {
      return Integer.parseInt(next());
    }

    long nextLong() {
      return Long.parseLong(next());
    }

    double nextDouble() { 
      return Double.parseDouble(next());
    }

    String nextLine() {
      String str = "";
        try{
            str = br.readLine();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return str;
      }
  }
}
