import java.util.Scanner;
public class Main{
    public static void main(String[] args) {


        Scanner sc = new Scanner(System.in);
        String str = sc.next(); 
        char[] c = str.toCharArray();

        int count = 0;
        int count1 = 0;
        int count2 = 0;

        char right1[] = new char[str.length()];
        for(int i = 0; i <c.length; i=i+2){
            right1[i] = '0';
        }
        for(int i = 1; i <c.length; i=i+2){
            right1[i] = '1';
        }

        char right2[] = new char[str.length()];
        for(int i = 0; i <c.length; i=i+2){
            right2[i] = '1';
        }
        for(int i = 1; i <c.length; i=i+2){
            right2[i] = '0';
        }

        for (int i = 0; i < c.length; i++) {
            if(c[i] != right1[i]){
                count1++;
            }
            if(c[i] != right2[i]){
                count2++;
            }
        }

        count = count1 <= count2 ? count1 : count2;
        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);
        String str = sc.next(); 
        char[] c = str.toCharArray();

        int count = 0;
        int count1 = 0;
        int count2 = 0;

        char right1[] = new char[str.length()];
        for(int i = 0; i <c.length; i=i+2){
            right1[i] = '0';
        }
        for(int i = 1; i <c.length; i=i+2){
            right1[i] = '1';
        }

        char right2[] = new char[str.length()];
        for(int i = 0; i <c.length; i=i+2){
            right2[i] = '1';
        }
        for(int i = 1; i <c.length; i=i+2){
            right2[i] = '0';
        }

        for (int i = 0; i < c.length; i++) {
            if(c[i] != right1[i]){
                count1++;
            }
            if(c[i] != right2[i]){
                count2++;
            }
        }

        count = count1 <= count2 ? count1 : count2;
        System.out.println(count);

    }
} Main public static void main(String[] args) {


        Scanner sc = new Scanner(System.in);
        String str = sc.next(); 
        char[] c = str.toCharArray();

        int count = 0;
        int count1 = 0;
        int count2 = 0;

        char right1[] = new char[str.length()];
        for(int i = 0; i <c.length; i=i+2){
            right1[i] = '0';
        }
        for(int i = 1; i <c.length; i=i+2){
            right1[i] = '1';
        }

        char right2[] = new char[str.length()];
        for(int i = 0; i <c.length; i=i+2){
            right2[i] = '1';
        }
        for(int i = 1; i <c.length; i=i+2){
            right2[i] = '0';
        }

        for (int i = 0; i < c.length; i++) {
            if(c[i] != right1[i]){
                count1++;
            }
            if(c[i] != right2[i]){
                count2++;
            }
        }

        count = count1 <= count2 ? count1 : count2;
        System.out.println(count);

    } main {


        Scanner sc = new Scanner(System.in);
        String str = sc.next(); 
        char[] c = str.toCharArray();

        int count = 0;
        int count1 = 0;
        int count2 = 0;

        char right1[] = new char[str.length()];
        for(int i = 0; i <c.length; i=i+2){
            right1[i] = '0';
        }
        for(int i = 1; i <c.length; i=i+2){
            right1[i] = '1';
        }

        char right2[] = new char[str.length()];
        for(int i = 0; i <c.length; i=i+2){
            right2[i] = '1';
        }
        for(int i = 1; i <c.length; i=i+2){
            right2[i] = '0';
        }

        for (int i = 0; i < c.length; i++) {
            if(c[i] != right1[i]){
                count1++;
            }
            if(c[i] != right2[i]){
                count2++;
            }
        }

        count = count1 <= count2 ? count1 : count2;
        System.out.println(count);

    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) String str = sc.next(); str sc.next() sc.next sc char[] c = str.toCharArray(); c str.toCharArray() str.toCharArray str int count = 0; count 0 int count1 = 0; count1 0 int count2 = 0; count2 0 char right1[] = new char[str.length()]; right1 new char[str.length()] str.length() str.length str for(int i = 0; i <c.length; i=i+2){
            right1[i] = '0';
        } int i = 0; int i = 0; i 0 i <c.length i c.length c c.length i=i+2 i=i+2 i i+2 i 2 {
            right1[i] = '0';
        } {
            right1[i] = '0';
        } right1[i] = '0' right1[i] right1 i '0' for(int i = 1; i <c.length; i=i+2){
            right1[i] = '1';
        } int i = 1; int i = 1; i 1 i <c.length i c.length c c.length i=i+2 i=i+2 i i+2 i 2 {
            right1[i] = '1';
        } {
            right1[i] = '1';
        } right1[i] = '1' right1[i] right1 i '1' char right2[] = new char[str.length()]; right2 new char[str.length()] str.length() str.length str for(int i = 0; i <c.length; i=i+2){
            right2[i] = '1';
        } int i = 0; int i = 0; i 0 i <c.length i c.length c c.length i=i+2 i=i+2 i i+2 i 2 {
            right2[i] = '1';
        } {
            right2[i] = '1';
        } right2[i] = '1' right2[i] right2 i '1' for(int i = 1; i <c.length; i=i+2){
            right2[i] = '0';
        } int i = 1; int i = 1; i 1 i <c.length i c.length c c.length i=i+2 i=i+2 i i+2 i 2 {
            right2[i] = '0';
        } {
            right2[i] = '0';
        } right2[i] = '0' right2[i] right2 i '0' for (int i = 0; i < c.length; i++) {
            if(c[i] != right1[i]){
                count1++;
            }
            if(c[i] != right2[i]){
                count2++;
            }
        } int i = 0; int i = 0; i 0 i < c.length i c.length c c.length i++ i++ i {
            if(c[i] != right1[i]){
                count1++;
            }
            if(c[i] != right2[i]){
                count2++;
            }
        } {
            if(c[i] != right1[i]){
                count1++;
            }
            if(c[i] != right2[i]){
                count2++;
            }
        } if(c[i] != right1[i]){
                count1++;
            } c[i] != right1[i] c[i] c i right1[i] right1 i {
                count1++;
            } count1++ count1 if(c[i] != right2[i]){
                count2++;
            } c[i] != right2[i] c[i] c i right2[i] right2 i {
                count2++;
            } count2++ count2 count = count1 <= count2 ? count1 : count2 count count1 <= count2 ? count1 : count2 count1 <= count2 count1 count2 count1 count2 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);
        String str = sc.next(); 
        char[] c = str.toCharArray();

        int count = 0;
        int count1 = 0;
        int count2 = 0;

        char right1[] = new char[str.length()];
        for(int i = 0; i <c.length; i=i+2){
            right1[i] = '0';
        }
        for(int i = 1; i <c.length; i=i+2){
            right1[i] = '1';
        }

        char right2[] = new char[str.length()];
        for(int i = 0; i <c.length; i=i+2){
            right2[i] = '1';
        }
        for(int i = 1; i <c.length; i=i+2){
            right2[i] = '0';
        }

        for (int i = 0; i < c.length; i++) {
            if(c[i] != right1[i]){
                count1++;
            }
            if(c[i] != right2[i]){
                count2++;
            }
        }

        count = count1 <= count2 ? count1 : count2;
        System.out.println(count);

    }
} public class Main{
    public static void main(String[] args) {


        Scanner sc = new Scanner(System.in);
        String str = sc.next(); 
        char[] c = str.toCharArray();

        int count = 0;
        int count1 = 0;
        int count2 = 0;

        char right1[] = new char[str.length()];
        for(int i = 0; i <c.length; i=i+2){
            right1[i] = '0';
        }
        for(int i = 1; i <c.length; i=i+2){
            right1[i] = '1';
        }

        char right2[] = new char[str.length()];
        for(int i = 0; i <c.length; i=i+2){
            right2[i] = '1';
        }
        for(int i = 1; i <c.length; i=i+2){
            right2[i] = '0';
        }

        for (int i = 0; i < c.length; i++) {
            if(c[i] != right1[i]){
                count1++;
            }
            if(c[i] != right2[i]){
                count2++;
            }
        }

        count = count1 <= count2 ? count1 : count2;
        System.out.println(count);

    }
} Main
import java.util.Scanner;
public class Main{
    public static void main(String[] args) {


        Scanner sc = new Scanner(System.in);
        String str = sc.next(); 
        char[] c = str.toCharArray();

        int count = 0;
        int count1 = 0;
        int count2 = 0;

        char right1[] = new char[str.length()];
        for(int i = 0; i <c.length; i=i+2){
            right1[i] = '0';
        }
        for(int i = 1; i <c.length; i=i+2){
            right1[i] = '1';
        }

        char right2[] = new char[str.length()];
        for(int i = 0; i <c.length; i=i+2){
            right2[i] = '1';
        }
        for(int i = 1; i <c.length; i=i+2){
            right2[i] = '0';
        }

        for (int i = 0; i < c.length; i++) {
            if(c[i] != right1[i]){
                count1++;
            }
            if(c[i] != right2[i]){
                count2++;
            }
        }

        count = count1 <= count2 ? count1 : count2;
        System.out.println(count);

    }
}
