import java.util.*;
public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);

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

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

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