import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.StringTokenizer;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.InputStream;

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

    static class CColoringColorfully {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            String x = in.next();
            StringBuilder a = new StringBuilder(x);
            StringBuilder b = new StringBuilder(x);
            int a1 = 0;
            int a2 = 0;
            for (int i = 1; i < x.length(); i++) {
                if (a.charAt(i) == a.charAt(i - 1)) {
                    if (a.charAt(i) == '1') a.setCharAt(i, '0');
                    else a.setCharAt(i, '1');
                    a1++;
                }
            }
            for (int i = x.length() - 1; i > 0; i--) {
                if (b.charAt(i) == b.charAt(i - 1)) {
                    if (b.charAt(i - 1) == '1') b.setCharAt(i - 1, '0');
                    else b.setCharAt(i - 1, '1');
                    a2++;
                }
            }
            out.println(Math.min(a1, a2));
        }

    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

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

    static class CColoringColorfully {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            String x = in.next();
            StringBuilder a = new StringBuilder(x);
            StringBuilder b = new StringBuilder(x);
            int a1 = 0;
            int a2 = 0;
            for (int i = 1; i < x.length(); i++) {
                if (a.charAt(i) == a.charAt(i - 1)) {
                    if (a.charAt(i) == '1') a.setCharAt(i, '0');
                    else a.setCharAt(i, '1');
                    a1++;
                }
            }
            for (int i = x.length() - 1; i > 0; i--) {
                if (b.charAt(i) == b.charAt(i - 1)) {
                    if (b.charAt(i - 1) == '1') b.setCharAt(i - 1, '0');
                    else b.setCharAt(i - 1, '1');
                    a2++;
                }
            }
            out.println(Math.min(a1, a2));
        }

    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

    }
} Main public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        CColoringColorfully solver = new CColoringColorfully();
        solver.solve(1, in, out);
        out.close();
    } main {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        CColoringColorfully solver = new CColoringColorfully();
        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 InputReader in = new InputReader(inputStream); in new InputReader(inputStream) PrintWriter out = new PrintWriter(outputStream); out new PrintWriter(outputStream) CColoringColorfully solver = new CColoringColorfully(); solver new CColoringColorfully() solver.solve(1, in, out) solver.solve solver 1 in out out.close() out.close out String[] args args static class CColoringColorfully {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            String x = in.next();
            StringBuilder a = new StringBuilder(x);
            StringBuilder b = new StringBuilder(x);
            int a1 = 0;
            int a2 = 0;
            for (int i = 1; i < x.length(); i++) {
                if (a.charAt(i) == a.charAt(i - 1)) {
                    if (a.charAt(i) == '1') a.setCharAt(i, '0');
                    else a.setCharAt(i, '1');
                    a1++;
                }
            }
            for (int i = x.length() - 1; i > 0; i--) {
                if (b.charAt(i) == b.charAt(i - 1)) {
                    if (b.charAt(i - 1) == '1') b.setCharAt(i - 1, '0');
                    else b.setCharAt(i - 1, '1');
                    a2++;
                }
            }
            out.println(Math.min(a1, a2));
        }

    } CColoringColorfully public void solve(int testNumber, InputReader in, PrintWriter out) {
            String x = in.next();
            StringBuilder a = new StringBuilder(x);
            StringBuilder b = new StringBuilder(x);
            int a1 = 0;
            int a2 = 0;
            for (int i = 1; i < x.length(); i++) {
                if (a.charAt(i) == a.charAt(i - 1)) {
                    if (a.charAt(i) == '1') a.setCharAt(i, '0');
                    else a.setCharAt(i, '1');
                    a1++;
                }
            }
            for (int i = x.length() - 1; i > 0; i--) {
                if (b.charAt(i) == b.charAt(i - 1)) {
                    if (b.charAt(i - 1) == '1') b.setCharAt(i - 1, '0');
                    else b.setCharAt(i - 1, '1');
                    a2++;
                }
            }
            out.println(Math.min(a1, a2));
        } solve {
            String x = in.next();
            StringBuilder a = new StringBuilder(x);
            StringBuilder b = new StringBuilder(x);
            int a1 = 0;
            int a2 = 0;
            for (int i = 1; i < x.length(); i++) {
                if (a.charAt(i) == a.charAt(i - 1)) {
                    if (a.charAt(i) == '1') a.setCharAt(i, '0');
                    else a.setCharAt(i, '1');
                    a1++;
                }
            }
            for (int i = x.length() - 1; i > 0; i--) {
                if (b.charAt(i) == b.charAt(i - 1)) {
                    if (b.charAt(i - 1) == '1') b.setCharAt(i - 1, '0');
                    else b.setCharAt(i - 1, '1');
                    a2++;
                }
            }
            out.println(Math.min(a1, a2));
        } String x = in.next(); x in.next() in.next in StringBuilder a = new StringBuilder(x); a new StringBuilder(x) StringBuilder b = new StringBuilder(x); b new StringBuilder(x) int a1 = 0; a1 0 int a2 = 0; a2 0 for (int i = 1; i < x.length(); i++) {
                if (a.charAt(i) == a.charAt(i - 1)) {
                    if (a.charAt(i) == '1') a.setCharAt(i, '0');
                    else a.setCharAt(i, '1');
                    a1++;
                }
            } int i = 1; int i = 1; i 1 i < x.length() i x.length() x.length x i++ i++ i {
                if (a.charAt(i) == a.charAt(i - 1)) {
                    if (a.charAt(i) == '1') a.setCharAt(i, '0');
                    else a.setCharAt(i, '1');
                    a1++;
                }
            } {
                if (a.charAt(i) == a.charAt(i - 1)) {
                    if (a.charAt(i) == '1') a.setCharAt(i, '0');
                    else a.setCharAt(i, '1');
                    a1++;
                }
            } if (a.charAt(i) == a.charAt(i - 1)) {
                    if (a.charAt(i) == '1') a.setCharAt(i, '0');
                    else a.setCharAt(i, '1');
                    a1++;
                } a.charAt(i) == a.charAt(i - 1) a.charAt(i) a.charAt a i a.charAt(i - 1) a.charAt a i - 1 i 1 {
                    if (a.charAt(i) == '1') a.setCharAt(i, '0');
                    else a.setCharAt(i, '1');
                    a1++;
                } if (a.charAt(i) == '1') a.setCharAt(i, '0');
                    else a.setCharAt(i, '1'); a.charAt(i) == '1' a.charAt(i) a.charAt a i '1' a.setCharAt(i, '0') a.setCharAt a i '0' a.setCharAt(i, '1') a.setCharAt a i '1' a1++ a1 for (int i = x.length() - 1; i > 0; i--) {
                if (b.charAt(i) == b.charAt(i - 1)) {
                    if (b.charAt(i - 1) == '1') b.setCharAt(i - 1, '0');
                    else b.setCharAt(i - 1, '1');
                    a2++;
                }
            } int i = x.length() - 1; int i = x.length() - 1; i x.length() - 1 x.length() x.length x 1 i > 0 i 0 i-- i-- i {
                if (b.charAt(i) == b.charAt(i - 1)) {
                    if (b.charAt(i - 1) == '1') b.setCharAt(i - 1, '0');
                    else b.setCharAt(i - 1, '1');
                    a2++;
                }
            } {
                if (b.charAt(i) == b.charAt(i - 1)) {
                    if (b.charAt(i - 1) == '1') b.setCharAt(i - 1, '0');
                    else b.setCharAt(i - 1, '1');
                    a2++;
                }
            } if (b.charAt(i) == b.charAt(i - 1)) {
                    if (b.charAt(i - 1) == '1') b.setCharAt(i - 1, '0');
                    else b.setCharAt(i - 1, '1');
                    a2++;
                } b.charAt(i) == b.charAt(i - 1) b.charAt(i) b.charAt b i b.charAt(i - 1) b.charAt b i - 1 i 1 {
                    if (b.charAt(i - 1) == '1') b.setCharAt(i - 1, '0');
                    else b.setCharAt(i - 1, '1');
                    a2++;
                } if (b.charAt(i - 1) == '1') b.setCharAt(i - 1, '0');
                    else b.setCharAt(i - 1, '1'); b.charAt(i - 1) == '1' b.charAt(i - 1) b.charAt b i - 1 i 1 '1' b.setCharAt(i - 1, '0') b.setCharAt b i - 1 i 1 '0' b.setCharAt(i - 1, '1') b.setCharAt b i - 1 i 1 '1' a2++ a2 out.println(Math.min(a1, a2)) out.println out Math.min(a1, a2) Math.min Math a1 a2 int testNumber testNumber InputReader in in PrintWriter out out static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

    } InputReader public BufferedReader reader; reader public StringTokenizer tokenizer; tokenizer public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        } InputReader {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        } reader = new BufferedReader(new InputStreamReader(stream), 32768) reader new BufferedReader(new InputStreamReader(stream), 32768) tokenizer = null tokenizer null InputStream stream stream public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        } next {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        } while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            } tokenizer == null || !tokenizer.hasMoreTokens() tokenizer == null tokenizer null !tokenizer.hasMoreTokens() tokenizer.hasMoreTokens() tokenizer.hasMoreTokens tokenizer {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            } try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                } catch (IOException e) {
                    throw new RuntimeException(e);
                } IOException e {
                    throw new RuntimeException(e);
                } throw new RuntimeException(e); new RuntimeException(e) {
                    tokenizer = new StringTokenizer(reader.readLine());
                } tokenizer = new StringTokenizer(reader.readLine()) tokenizer new StringTokenizer(reader.readLine()) return tokenizer.nextToken(); tokenizer.nextToken() tokenizer.nextToken tokenizer public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        CColoringColorfully solver = new CColoringColorfully();
        solver.solve(1, in, out);
        out.close();
    }

    static class CColoringColorfully {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            String x = in.next();
            StringBuilder a = new StringBuilder(x);
            StringBuilder b = new StringBuilder(x);
            int a1 = 0;
            int a2 = 0;
            for (int i = 1; i < x.length(); i++) {
                if (a.charAt(i) == a.charAt(i - 1)) {
                    if (a.charAt(i) == '1') a.setCharAt(i, '0');
                    else a.setCharAt(i, '1');
                    a1++;
                }
            }
            for (int i = x.length() - 1; i > 0; i--) {
                if (b.charAt(i) == b.charAt(i - 1)) {
                    if (b.charAt(i - 1) == '1') b.setCharAt(i - 1, '0');
                    else b.setCharAt(i - 1, '1');
                    a2++;
                }
            }
            out.println(Math.min(a1, a2));
        }

    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

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

    static class CColoringColorfully {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            String x = in.next();
            StringBuilder a = new StringBuilder(x);
            StringBuilder b = new StringBuilder(x);
            int a1 = 0;
            int a2 = 0;
            for (int i = 1; i < x.length(); i++) {
                if (a.charAt(i) == a.charAt(i - 1)) {
                    if (a.charAt(i) == '1') a.setCharAt(i, '0');
                    else a.setCharAt(i, '1');
                    a1++;
                }
            }
            for (int i = x.length() - 1; i > 0; i--) {
                if (b.charAt(i) == b.charAt(i - 1)) {
                    if (b.charAt(i - 1) == '1') b.setCharAt(i - 1, '0');
                    else b.setCharAt(i - 1, '1');
                    a2++;
                }
            }
            out.println(Math.min(a1, a2));
        }

    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

    }
} Main
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.StringTokenizer;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.InputStream;

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

    static class CColoringColorfully {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            String x = in.next();
            StringBuilder a = new StringBuilder(x);
            StringBuilder b = new StringBuilder(x);
            int a1 = 0;
            int a2 = 0;
            for (int i = 1; i < x.length(); i++) {
                if (a.charAt(i) == a.charAt(i - 1)) {
                    if (a.charAt(i) == '1') a.setCharAt(i, '0');
                    else a.setCharAt(i, '1');
                    a1++;
                }
            }
            for (int i = x.length() - 1; i > 0; i--) {
                if (b.charAt(i) == b.charAt(i - 1)) {
                    if (b.charAt(i - 1) == '1') b.setCharAt(i - 1, '0');
                    else b.setCharAt(i - 1, '1');
                    a2++;
                }
            }
            out.println(Math.min(a1, a2));
        }

    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

    }
}

