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 anand.oza
 */
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();

            int n = s.length();

            int[] c = new int[n];
            for (int i = 0; i < n; i++) {
                c[i] = s.charAt(i) - '0';
            }

            int ans = Integer.MAX_VALUE;

            for (int parity = 0; parity <= 1; parity++) {
                int score = 0;

                for (int i = 0; i < n; i++) {
                    score += (c[i] + i + parity) % 2;
                }

                ans = Math.min(ans, score);
            }

            out.println(ans);
        }

    }

    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();

            int n = s.length();

            int[] c = new int[n];
            for (int i = 0; i < n; i++) {
                c[i] = s.charAt(i) - '0';
            }

            int ans = Integer.MAX_VALUE;

            for (int parity = 0; parity <= 1; parity++) {
                int score = 0;

                for (int i = 0; i < n; i++) {
                    score += (c[i] + i + parity) % 2;
                }

                ans = Math.min(ans, score);
            }

            out.println(ans);
        }

    }

    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();

            int n = s.length();

            int[] c = new int[n];
            for (int i = 0; i < n; i++) {
                c[i] = s.charAt(i) - '0';
            }

            int ans = Integer.MAX_VALUE;

            for (int parity = 0; parity <= 1; parity++) {
                int score = 0;

                for (int i = 0; i < n; i++) {
                    score += (c[i] + i + parity) % 2;
                }

                ans = Math.min(ans, score);
            }

            out.println(ans);
        }

    } CColoringColorfully public void solve(int testNumber, InputReader in, PrintWriter out) {
            String s = in.next();

            int n = s.length();

            int[] c = new int[n];
            for (int i = 0; i < n; i++) {
                c[i] = s.charAt(i) - '0';
            }

            int ans = Integer.MAX_VALUE;

            for (int parity = 0; parity <= 1; parity++) {
                int score = 0;

                for (int i = 0; i < n; i++) {
                    score += (c[i] + i + parity) % 2;
                }

                ans = Math.min(ans, score);
            }

            out.println(ans);
        } solve {
            String s = in.next();

            int n = s.length();

            int[] c = new int[n];
            for (int i = 0; i < n; i++) {
                c[i] = s.charAt(i) - '0';
            }

            int ans = Integer.MAX_VALUE;

            for (int parity = 0; parity <= 1; parity++) {
                int score = 0;

                for (int i = 0; i < n; i++) {
                    score += (c[i] + i + parity) % 2;
                }

                ans = Math.min(ans, score);
            }

            out.println(ans);
        } String s = in.next(); s in.next() in.next in int n = s.length(); n s.length() s.length s int[] c = new int[n]; c new int[n] n for (int i = 0; i < n; i++) {
                c[i] = s.charAt(i) - '0';
            } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
                c[i] = s.charAt(i) - '0';
            } {
                c[i] = s.charAt(i) - '0';
            } c[i] = s.charAt(i) - '0' c[i] c i s.charAt(i) - '0' s.charAt(i) s.charAt s i '0' int ans = Integer.MAX_VALUE; ans Integer.MAX_VALUE Integer Integer.MAX_VALUE for (int parity = 0; parity <= 1; parity++) {
                int score = 0;

                for (int i = 0; i < n; i++) {
                    score += (c[i] + i + parity) % 2;
                }

                ans = Math.min(ans, score);
            } int parity = 0; int parity = 0; parity 0 parity <= 1 parity 1 parity++ parity++ parity {
                int score = 0;

                for (int i = 0; i < n; i++) {
                    score += (c[i] + i + parity) % 2;
                }

                ans = Math.min(ans, score);
            } {
                int score = 0;

                for (int i = 0; i < n; i++) {
                    score += (c[i] + i + parity) % 2;
                }

                ans = Math.min(ans, score);
            } int score = 0; score 0 for (int i = 0; i < n; i++) {
                    score += (c[i] + i + parity) % 2;
                } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
                    score += (c[i] + i + parity) % 2;
                } {
                    score += (c[i] + i + parity) % 2;
                } score += (c[i] + i + parity) % 2 score (c[i] + i + parity) % 2 (c[i] + i + parity) c[i] + i + parity c[i] c i i parity 2 ans = Math.min(ans, score) ans Math.min(ans, score) Math.min Math ans score out.println(ans) out.println out ans 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();

            int n = s.length();

            int[] c = new int[n];
            for (int i = 0; i < n; i++) {
                c[i] = s.charAt(i) - '0';
            }

            int ans = Integer.MAX_VALUE;

            for (int parity = 0; parity <= 1; parity++) {
                int score = 0;

                for (int i = 0; i < n; i++) {
                    score += (c[i] + i + parity) % 2;
                }

                ans = Math.min(ans, score);
            }

            out.println(ans);
        }

    }

    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();

            int n = s.length();

            int[] c = new int[n];
            for (int i = 0; i < n; i++) {
                c[i] = s.charAt(i) - '0';
            }

            int ans = Integer.MAX_VALUE;

            for (int parity = 0; parity <= 1; parity++) {
                int score = 0;

                for (int i = 0; i < n; i++) {
                    score += (c[i] + i + parity) % 2;
                }

                ans = Math.min(ans, score);
            }

            out.println(ans);
        }

    }

    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 anand.oza
 */
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();

            int n = s.length();

            int[] c = new int[n];
            for (int i = 0; i < n; i++) {
                c[i] = s.charAt(i) - '0';
            }

            int ans = Integer.MAX_VALUE;

            for (int parity = 0; parity <= 1; parity++) {
                int score = 0;

                for (int i = 0; i < n; i++) {
                    score += (c[i] + i + parity) % 2;
                }

                ans = Math.min(ans, score);
            }

            out.println(ans);
        }

    }

    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();
        }

    }
}

