
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		String S = sc.nextLine();
		String[] Ss = S.split("");
		int count = 0;

		if(Ss.length == 1) {
			System.out.println(0);
		}else {
			for(int i = 0; i < Ss.length-1 ; i++) {
				if(Ss[i].equals(Ss[i+1])) {
					count++;
					if(Ss[i+1].equals("0")) {
						Ss[i+1] = "1";
					}else {
						Ss[i+1] = "0";
					}
				}
			}
			System.out.println(count);
		}

	}

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

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		String S = sc.nextLine();
		String[] Ss = S.split("");
		int count = 0;

		if(Ss.length == 1) {
			System.out.println(0);
		}else {
			for(int i = 0; i < Ss.length-1 ; i++) {
				if(Ss[i].equals(Ss[i+1])) {
					count++;
					if(Ss[i+1].equals("0")) {
						Ss[i+1] = "1";
					}else {
						Ss[i+1] = "0";
					}
				}
			}
			System.out.println(count);
		}

	}

} Main public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		String S = sc.nextLine();
		String[] Ss = S.split("");
		int count = 0;

		if(Ss.length == 1) {
			System.out.println(0);
		}else {
			for(int i = 0; i < Ss.length-1 ; i++) {
				if(Ss[i].equals(Ss[i+1])) {
					count++;
					if(Ss[i+1].equals("0")) {
						Ss[i+1] = "1";
					}else {
						Ss[i+1] = "0";
					}
				}
			}
			System.out.println(count);
		}

	} main {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		String S = sc.nextLine();
		String[] Ss = S.split("");
		int count = 0;

		if(Ss.length == 1) {
			System.out.println(0);
		}else {
			for(int i = 0; i < Ss.length-1 ; i++) {
				if(Ss[i].equals(Ss[i+1])) {
					count++;
					if(Ss[i+1].equals("0")) {
						Ss[i+1] = "1";
					}else {
						Ss[i+1] = "0";
					}
				}
			}
			System.out.println(count);
		}

	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) String S = sc.nextLine(); S sc.nextLine() sc.nextLine sc String[] Ss = S.split(""); Ss S.split("") S.split S "" int count = 0; count 0 if(Ss.length == 1) {
			System.out.println(0);
		}else {
			for(int i = 0; i < Ss.length-1 ; i++) {
				if(Ss[i].equals(Ss[i+1])) {
					count++;
					if(Ss[i+1].equals("0")) {
						Ss[i+1] = "1";
					}else {
						Ss[i+1] = "0";
					}
				}
			}
			System.out.println(count);
		} Ss.length == 1 Ss.length Ss Ss.length 1 {
			System.out.println(0);
		} System.out.println(0) System.out.println System.out System System.out 0 {
			for(int i = 0; i < Ss.length-1 ; i++) {
				if(Ss[i].equals(Ss[i+1])) {
					count++;
					if(Ss[i+1].equals("0")) {
						Ss[i+1] = "1";
					}else {
						Ss[i+1] = "0";
					}
				}
			}
			System.out.println(count);
		} for(int i = 0; i < Ss.length-1 ; i++) {
				if(Ss[i].equals(Ss[i+1])) {
					count++;
					if(Ss[i+1].equals("0")) {
						Ss[i+1] = "1";
					}else {
						Ss[i+1] = "0";
					}
				}
			} int i = 0; int i = 0; i 0 i < Ss.length-1 i Ss.length-1 Ss.length Ss Ss.length 1 i++ i++ i {
				if(Ss[i].equals(Ss[i+1])) {
					count++;
					if(Ss[i+1].equals("0")) {
						Ss[i+1] = "1";
					}else {
						Ss[i+1] = "0";
					}
				}
			} {
				if(Ss[i].equals(Ss[i+1])) {
					count++;
					if(Ss[i+1].equals("0")) {
						Ss[i+1] = "1";
					}else {
						Ss[i+1] = "0";
					}
				}
			} if(Ss[i].equals(Ss[i+1])) {
					count++;
					if(Ss[i+1].equals("0")) {
						Ss[i+1] = "1";
					}else {
						Ss[i+1] = "0";
					}
				} Ss[i].equals(Ss[i+1]) Ss[i].equals Ss[i] Ss i Ss[i+1] Ss i+1 i 1 {
					count++;
					if(Ss[i+1].equals("0")) {
						Ss[i+1] = "1";
					}else {
						Ss[i+1] = "0";
					}
				} count++ count if(Ss[i+1].equals("0")) {
						Ss[i+1] = "1";
					}else {
						Ss[i+1] = "0";
					} Ss[i+1].equals("0") Ss[i+1].equals Ss[i+1] Ss i+1 i 1 "0" {
						Ss[i+1] = "1";
					} Ss[i+1] = "1" Ss[i+1] Ss i+1 i 1 "1" {
						Ss[i+1] = "0";
					} Ss[i+1] = "0" Ss[i+1] Ss i+1 i 1 "0" 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) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		String S = sc.nextLine();
		String[] Ss = S.split("");
		int count = 0;

		if(Ss.length == 1) {
			System.out.println(0);
		}else {
			for(int i = 0; i < Ss.length-1 ; i++) {
				if(Ss[i].equals(Ss[i+1])) {
					count++;
					if(Ss[i+1].equals("0")) {
						Ss[i+1] = "1";
					}else {
						Ss[i+1] = "0";
					}
				}
			}
			System.out.println(count);
		}

	}

} public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		String S = sc.nextLine();
		String[] Ss = S.split("");
		int count = 0;

		if(Ss.length == 1) {
			System.out.println(0);
		}else {
			for(int i = 0; i < Ss.length-1 ; i++) {
				if(Ss[i].equals(Ss[i+1])) {
					count++;
					if(Ss[i+1].equals("0")) {
						Ss[i+1] = "1";
					}else {
						Ss[i+1] = "0";
					}
				}
			}
			System.out.println(count);
		}

	}

} Main

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		String S = sc.nextLine();
		String[] Ss = S.split("");
		int count = 0;

		if(Ss.length == 1) {
			System.out.println(0);
		}else {
			for(int i = 0; i < Ss.length-1 ; i++) {
				if(Ss[i].equals(Ss[i+1])) {
					count++;
					if(Ss[i+1].equals("0")) {
						Ss[i+1] = "1";
					}else {
						Ss[i+1] = "0";
					}
				}
			}
			System.out.println(count);
		}

	}

}
