import java.util.Scanner;
public class Main{

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		char []s = sc.next().toCharArray();
		int p = s.length;
		int []t = new int [p];
		
		for(int i = 0; i< p;i++) {
			if(s[i] == '0') {
				t[i] = 0;
			}
			if(s[i] == '1') {
				t[i] = 1;
			}
		}
		int count = 0;
		for(int i = 0 ; i < p-1 ; i++) {
			if(t[i] + t[i+1] == 0) {
				t[i+1] = 1;
				count++;
			}
			if(t[i] + t[i+1] == 2) {
				t[i+1] = 0;
				count++;
			}
			
		}
		System.out.println(count);
		
	}

} import java.util.Scanner; Scanner java.util public class Main{

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		char []s = sc.next().toCharArray();
		int p = s.length;
		int []t = new int [p];
		
		for(int i = 0; i< p;i++) {
			if(s[i] == '0') {
				t[i] = 0;
			}
			if(s[i] == '1') {
				t[i] = 1;
			}
		}
		int count = 0;
		for(int i = 0 ; i < p-1 ; i++) {
			if(t[i] + t[i+1] == 0) {
				t[i+1] = 1;
				count++;
			}
			if(t[i] + t[i+1] == 2) {
				t[i+1] = 0;
				count++;
			}
			
		}
		System.out.println(count);
		
	}

} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		char []s = sc.next().toCharArray();
		int p = s.length;
		int []t = new int [p];
		
		for(int i = 0; i< p;i++) {
			if(s[i] == '0') {
				t[i] = 0;
			}
			if(s[i] == '1') {
				t[i] = 1;
			}
		}
		int count = 0;
		for(int i = 0 ; i < p-1 ; i++) {
			if(t[i] + t[i+1] == 0) {
				t[i+1] = 1;
				count++;
			}
			if(t[i] + t[i+1] == 2) {
				t[i+1] = 0;
				count++;
			}
			
		}
		System.out.println(count);
		
	} main {
		Scanner sc = new Scanner(System.in);
		
		char []s = sc.next().toCharArray();
		int p = s.length;
		int []t = new int [p];
		
		for(int i = 0; i< p;i++) {
			if(s[i] == '0') {
				t[i] = 0;
			}
			if(s[i] == '1') {
				t[i] = 1;
			}
		}
		int count = 0;
		for(int i = 0 ; i < p-1 ; i++) {
			if(t[i] + t[i+1] == 0) {
				t[i+1] = 1;
				count++;
			}
			if(t[i] + t[i+1] == 2) {
				t[i+1] = 0;
				count++;
			}
			
		}
		System.out.println(count);
		
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) char []s = sc.next().toCharArray(); s sc.next().toCharArray() sc.next().toCharArray sc.next() sc.next sc int p = s.length; p s.length s s.length int []t = new int [p]; t new int [p] p for(int i = 0; i< p;i++) {
			if(s[i] == '0') {
				t[i] = 0;
			}
			if(s[i] == '1') {
				t[i] = 1;
			}
		} int i = 0; int i = 0; i 0 i< p i p i++ i++ i {
			if(s[i] == '0') {
				t[i] = 0;
			}
			if(s[i] == '1') {
				t[i] = 1;
			}
		} {
			if(s[i] == '0') {
				t[i] = 0;
			}
			if(s[i] == '1') {
				t[i] = 1;
			}
		} if(s[i] == '0') {
				t[i] = 0;
			} s[i] == '0' s[i] s i '0' {
				t[i] = 0;
			} t[i] = 0 t[i] t i 0 if(s[i] == '1') {
				t[i] = 1;
			} s[i] == '1' s[i] s i '1' {
				t[i] = 1;
			} t[i] = 1 t[i] t i 1 int count = 0; count 0 for(int i = 0 ; i < p-1 ; i++) {
			if(t[i] + t[i+1] == 0) {
				t[i+1] = 1;
				count++;
			}
			if(t[i] + t[i+1] == 2) {
				t[i+1] = 0;
				count++;
			}
			
		} int i = 0 ; int i = 0 ; i 0 i < p-1 i p-1 p 1 i++ i++ i {
			if(t[i] + t[i+1] == 0) {
				t[i+1] = 1;
				count++;
			}
			if(t[i] + t[i+1] == 2) {
				t[i+1] = 0;
				count++;
			}
			
		} {
			if(t[i] + t[i+1] == 0) {
				t[i+1] = 1;
				count++;
			}
			if(t[i] + t[i+1] == 2) {
				t[i+1] = 0;
				count++;
			}
			
		} if(t[i] + t[i+1] == 0) {
				t[i+1] = 1;
				count++;
			} t[i] + t[i+1] == 0 t[i] + t[i+1] t[i] t i t[i+1] t i+1 i 1 0 {
				t[i+1] = 1;
				count++;
			} t[i+1] = 1 t[i+1] t i+1 i 1 1 count++ count if(t[i] + t[i+1] == 2) {
				t[i+1] = 0;
				count++;
			} t[i] + t[i+1] == 2 t[i] + t[i+1] t[i] t i t[i+1] t i+1 i 1 2 {
				t[i+1] = 0;
				count++;
			} t[i+1] = 0 t[i+1] t i+1 i 1 0 count++ count System.out.println(count) System.out.println System.out System System.out count String[] args args public class Main{

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		char []s = sc.next().toCharArray();
		int p = s.length;
		int []t = new int [p];
		
		for(int i = 0; i< p;i++) {
			if(s[i] == '0') {
				t[i] = 0;
			}
			if(s[i] == '1') {
				t[i] = 1;
			}
		}
		int count = 0;
		for(int i = 0 ; i < p-1 ; i++) {
			if(t[i] + t[i+1] == 0) {
				t[i+1] = 1;
				count++;
			}
			if(t[i] + t[i+1] == 2) {
				t[i+1] = 0;
				count++;
			}
			
		}
		System.out.println(count);
		
	}

} public class Main{

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		char []s = sc.next().toCharArray();
		int p = s.length;
		int []t = new int [p];
		
		for(int i = 0; i< p;i++) {
			if(s[i] == '0') {
				t[i] = 0;
			}
			if(s[i] == '1') {
				t[i] = 1;
			}
		}
		int count = 0;
		for(int i = 0 ; i < p-1 ; i++) {
			if(t[i] + t[i+1] == 0) {
				t[i+1] = 1;
				count++;
			}
			if(t[i] + t[i+1] == 2) {
				t[i+1] = 0;
				count++;
			}
			
		}
		System.out.println(count);
		
	}

} Main
import java.util.Scanner;
public class Main{

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		char []s = sc.next().toCharArray();
		int p = s.length;
		int []t = new int [p];
		
		for(int i = 0; i< p;i++) {
			if(s[i] == '0') {
				t[i] = 0;
			}
			if(s[i] == '1') {
				t[i] = 1;
			}
		}
		int count = 0;
		for(int i = 0 ; i < p-1 ; i++) {
			if(t[i] + t[i+1] == 0) {
				t[i+1] = 1;
				count++;
			}
			if(t[i] + t[i+1] == 2) {
				t[i+1] = 0;
				count++;
			}
			
		}
		System.out.println(count);
		
	}

}
