import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        C solver = new C();
        solver.solve(1, in, out);
        out.close();
    }

    static class C {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            String S = in.next();
            int b1 = 0, b2 = 0, len = S.length();
            for (int i = 0; i < len; i++) {
                if (i % 2 == 0) {
                    if (S.charAt(i) == '0') {
                        b2++;
                    } else {
                        b1++;
                    }
                }
                if (i % 2 == 1) {
                    if (S.charAt(i) == '0') {
                        b1++;
                    } else {
                        b2++;
                    }
                }
            }
            out.println(Math.min(b1, b2));
        }

    }
}
 import java.io.OutputStream; OutputStream java.io import java.io.IOException; IOException java.io import java.io.InputStream; InputStream java.io import java.io.PrintWriter; PrintWriter java.io import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        C solver = new C();
        solver.solve(1, in, out);
        out.close();
    }

    static class C {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            String S = in.next();
            int b1 = 0, b2 = 0, len = S.length();
            for (int i = 0; i < len; i++) {
                if (i % 2 == 0) {
                    if (S.charAt(i) == '0') {
                        b2++;
                    } else {
                        b1++;
                    }
                }
                if (i % 2 == 1) {
                    if (S.charAt(i) == '0') {
                        b1++;
                    } else {
                        b2++;
                    }
                }
            }
            out.println(Math.min(b1, b2));
        }

    }
} Main public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        C solver = new C();
        solver.solve(1, in, out);
        out.close();
    } main {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        C solver = new C();
        solver.solve(1, in, out);
        out.close();
    } InputStream inputStream = System.in; inputStream System.in System System.in OutputStream outputStream = System.out; outputStream System.out System System.out Scanner in = new Scanner(inputStream); in new Scanner(inputStream) PrintWriter out = new PrintWriter(outputStream); out new PrintWriter(outputStream) C solver = new C(); solver new C() solver.solve(1, in, out) solver.solve solver 1 in out out.close() out.close out String[] args args static class C {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            String S = in.next();
            int b1 = 0, b2 = 0, len = S.length();
            for (int i = 0; i < len; i++) {
                if (i % 2 == 0) {
                    if (S.charAt(i) == '0') {
                        b2++;
                    } else {
                        b1++;
                    }
                }
                if (i % 2 == 1) {
                    if (S.charAt(i) == '0') {
                        b1++;
                    } else {
                        b2++;
                    }
                }
            }
            out.println(Math.min(b1, b2));
        }

    } C public void solve(int testNumber, Scanner in, PrintWriter out) {
            String S = in.next();
            int b1 = 0, b2 = 0, len = S.length();
            for (int i = 0; i < len; i++) {
                if (i % 2 == 0) {
                    if (S.charAt(i) == '0') {
                        b2++;
                    } else {
                        b1++;
                    }
                }
                if (i % 2 == 1) {
                    if (S.charAt(i) == '0') {
                        b1++;
                    } else {
                        b2++;
                    }
                }
            }
            out.println(Math.min(b1, b2));
        } solve {
            String S = in.next();
            int b1 = 0, b2 = 0, len = S.length();
            for (int i = 0; i < len; i++) {
                if (i % 2 == 0) {
                    if (S.charAt(i) == '0') {
                        b2++;
                    } else {
                        b1++;
                    }
                }
                if (i % 2 == 1) {
                    if (S.charAt(i) == '0') {
                        b1++;
                    } else {
                        b2++;
                    }
                }
            }
            out.println(Math.min(b1, b2));
        } String S = in.next(); S in.next() in.next in int b1 = 0 b1 0 b2 = 0 b2 0 len = S.length(); len S.length() S.length S for (int i = 0; i < len; i++) {
                if (i % 2 == 0) {
                    if (S.charAt(i) == '0') {
                        b2++;
                    } else {
                        b1++;
                    }
                }
                if (i % 2 == 1) {
                    if (S.charAt(i) == '0') {
                        b1++;
                    } else {
                        b2++;
                    }
                }
            } int i = 0; int i = 0; i 0 i < len i len i++ i++ i {
                if (i % 2 == 0) {
                    if (S.charAt(i) == '0') {
                        b2++;
                    } else {
                        b1++;
                    }
                }
                if (i % 2 == 1) {
                    if (S.charAt(i) == '0') {
                        b1++;
                    } else {
                        b2++;
                    }
                }
            } {
                if (i % 2 == 0) {
                    if (S.charAt(i) == '0') {
                        b2++;
                    } else {
                        b1++;
                    }
                }
                if (i % 2 == 1) {
                    if (S.charAt(i) == '0') {
                        b1++;
                    } else {
                        b2++;
                    }
                }
            } if (i % 2 == 0) {
                    if (S.charAt(i) == '0') {
                        b2++;
                    } else {
                        b1++;
                    }
                } i % 2 == 0 i % 2 i 2 0 {
                    if (S.charAt(i) == '0') {
                        b2++;
                    } else {
                        b1++;
                    }
                } if (S.charAt(i) == '0') {
                        b2++;
                    } else {
                        b1++;
                    } S.charAt(i) == '0' S.charAt(i) S.charAt S i '0' {
                        b2++;
                    } b2++ b2 {
                        b1++;
                    } b1++ b1 if (i % 2 == 1) {
                    if (S.charAt(i) == '0') {
                        b1++;
                    } else {
                        b2++;
                    }
                } i % 2 == 1 i % 2 i 2 1 {
                    if (S.charAt(i) == '0') {
                        b1++;
                    } else {
                        b2++;
                    }
                } if (S.charAt(i) == '0') {
                        b1++;
                    } else {
                        b2++;
                    } S.charAt(i) == '0' S.charAt(i) S.charAt S i '0' {
                        b1++;
                    } b1++ b1 {
                        b2++;
                    } b2++ b2 out.println(Math.min(b1, b2)) out.println out Math.min(b1, b2) Math.min Math b1 b2 int testNumber testNumber Scanner in in PrintWriter out out public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        C solver = new C();
        solver.solve(1, in, out);
        out.close();
    }

    static class C {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            String S = in.next();
            int b1 = 0, b2 = 0, len = S.length();
            for (int i = 0; i < len; i++) {
                if (i % 2 == 0) {
                    if (S.charAt(i) == '0') {
                        b2++;
                    } else {
                        b1++;
                    }
                }
                if (i % 2 == 1) {
                    if (S.charAt(i) == '0') {
                        b1++;
                    } else {
                        b2++;
                    }
                }
            }
            out.println(Math.min(b1, b2));
        }

    }
} public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        C solver = new C();
        solver.solve(1, in, out);
        out.close();
    }

    static class C {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            String S = in.next();
            int b1 = 0, b2 = 0, len = S.length();
            for (int i = 0; i < len; i++) {
                if (i % 2 == 0) {
                    if (S.charAt(i) == '0') {
                        b2++;
                    } else {
                        b1++;
                    }
                }
                if (i % 2 == 1) {
                    if (S.charAt(i) == '0') {
                        b1++;
                    } else {
                        b2++;
                    }
                }
            }
            out.println(Math.min(b1, b2));
        }

    }
} Main
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        C solver = new C();
        solver.solve(1, in, out);
        out.close();
    }

    static class C {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            String S = in.next();
            int b1 = 0, b2 = 0, len = S.length();
            for (int i = 0; i < len; i++) {
                if (i % 2 == 0) {
                    if (S.charAt(i) == '0') {
                        b2++;
                    } else {
                        b1++;
                    }
                }
                if (i % 2 == 1) {
                    if (S.charAt(i) == '0') {
                        b1++;
                    } else {
                        b2++;
                    }
                }
            }
            out.println(Math.min(b1, b2));
        }

    }
}

