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
 */
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 s = in.next();
            boolean fl = true;
            int changes1 = 0, changes2 = 0;
            for (int i = 0, l = s.length(); i < l; i++, fl = !fl) {
                char cur = s.charAt(i);
                if (fl) {
                    if (cur == '0')
                        changes1++;
                    else
                        changes2++;
                } else {
                    if (cur == '1')
                        changes1++;
                    else
                        changes2++;
                }
            }
            out.println(Math.min(changes1, changes2));
        }

    }

    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 s = in.next();
            boolean fl = true;
            int changes1 = 0, changes2 = 0;
            for (int i = 0, l = s.length(); i < l; i++, fl = !fl) {
                char cur = s.charAt(i);
                if (fl) {
                    if (cur == '0')
                        changes1++;
                    else
                        changes2++;
                } else {
                    if (cur == '1')
                        changes1++;
                    else
                        changes2++;
                }
            }
            out.println(Math.min(changes1, changes2));
        }

    }

    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 s = in.next();
            boolean fl = true;
            int changes1 = 0, changes2 = 0;
            for (int i = 0, l = s.length(); i < l; i++, fl = !fl) {
                char cur = s.charAt(i);
                if (fl) {
                    if (cur == '0')
                        changes1++;
                    else
                        changes2++;
                } else {
                    if (cur == '1')
                        changes1++;
                    else
                        changes2++;
                }
            }
            out.println(Math.min(changes1, changes2));
        }

    } CColoringColorfully public void solve(int testNumber, InputReader in, PrintWriter out) {
            String s = in.next();
            boolean fl = true;
            int changes1 = 0, changes2 = 0;
            for (int i = 0, l = s.length(); i < l; i++, fl = !fl) {
                char cur = s.charAt(i);
                if (fl) {
                    if (cur == '0')
                        changes1++;
                    else
                        changes2++;
                } else {
                    if (cur == '1')
                        changes1++;
                    else
                        changes2++;
                }
            }
            out.println(Math.min(changes1, changes2));
        } solve {
            String s = in.next();
            boolean fl = true;
            int changes1 = 0, changes2 = 0;
            for (int i = 0, l = s.length(); i < l; i++, fl = !fl) {
                char cur = s.charAt(i);
                if (fl) {
                    if (cur == '0')
                        changes1++;
                    else
                        changes2++;
                } else {
                    if (cur == '1')
                        changes1++;
                    else
                        changes2++;
                }
            }
            out.println(Math.min(changes1, changes2));
        } String s = in.next(); s in.next() in.next in boolean fl = true; fl true int changes1 = 0 changes1 0 changes2 = 0; changes2 0 for (int i = 0, l = s.length(); i < l; i++, fl = !fl) {
                char cur = s.charAt(i);
                if (fl) {
                    if (cur == '0')
                        changes1++;
                    else
                        changes2++;
                } else {
                    if (cur == '1')
                        changes1++;
                    else
                        changes2++;
                }
            } int i = 0, l = s.length(); int i = 0 i 0 l = s.length(); l s.length() s.length s i < l i l i++, fl = !fl i++ i fl = !fl fl !fl fl {
                char cur = s.charAt(i);
                if (fl) {
                    if (cur == '0')
                        changes1++;
                    else
                        changes2++;
                } else {
                    if (cur == '1')
                        changes1++;
                    else
                        changes2++;
                }
            } {
                char cur = s.charAt(i);
                if (fl) {
                    if (cur == '0')
                        changes1++;
                    else
                        changes2++;
                } else {
                    if (cur == '1')
                        changes1++;
                    else
                        changes2++;
                }
            } char cur = s.charAt(i); cur s.charAt(i) s.charAt s i if (fl) {
                    if (cur == '0')
                        changes1++;
                    else
                        changes2++;
                } else {
                    if (cur == '1')
                        changes1++;
                    else
                        changes2++;
                } fl {
                    if (cur == '0')
                        changes1++;
                    else
                        changes2++;
                } if (cur == '0')
                        changes1++;
                    else
                        changes2++; cur == '0' cur '0' changes1++ changes1 changes2++ changes2 {
                    if (cur == '1')
                        changes1++;
                    else
                        changes2++;
                } if (cur == '1')
                        changes1++;
                    else
                        changes2++; cur == '1' cur '1' changes1++ changes1 changes2++ changes2 out.println(Math.min(changes1, changes2)) out.println out Math.min(changes1, changes2) Math.min Math changes1 changes2 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 s = in.next();
            boolean fl = true;
            int changes1 = 0, changes2 = 0;
            for (int i = 0, l = s.length(); i < l; i++, fl = !fl) {
                char cur = s.charAt(i);
                if (fl) {
                    if (cur == '0')
                        changes1++;
                    else
                        changes2++;
                } else {
                    if (cur == '1')
                        changes1++;
                    else
                        changes2++;
                }
            }
            out.println(Math.min(changes1, changes2));
        }

    }

    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 s = in.next();
            boolean fl = true;
            int changes1 = 0, changes2 = 0;
            for (int i = 0, l = s.length(); i < l; i++, fl = !fl) {
                char cur = s.charAt(i);
                if (fl) {
                    if (cur == '0')
                        changes1++;
                    else
                        changes2++;
                } else {
                    if (cur == '1')
                        changes1++;
                    else
                        changes2++;
                }
            }
            out.println(Math.min(changes1, changes2));
        }

    }

    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
 */
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 s = in.next();
            boolean fl = true;
            int changes1 = 0, changes2 = 0;
            for (int i = 0, l = s.length(); i < l; i++, fl = !fl) {
                char cur = s.charAt(i);
                if (fl) {
                    if (cur == '0')
                        changes1++;
                    else
                        changes2++;
                } else {
                    if (cur == '1')
                        changes1++;
                    else
                        changes2++;
                }
            }
            out.println(Math.min(changes1, changes2));
        }

    }

    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();
        }

    }
}