import java.io.*;
import java.util.StringTokenizer;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        PrintWriter out = new PrintWriter(new OutputStreamWriter(System.out));

        String str = in.nextLine();
        StringBuilder sb1 = new StringBuilder();
        StringBuilder sb2 = new StringBuilder();
        for (int i=0; i<str.length(); i++) {
            if ((i & 1) == 0) {
                sb1.append('0');
            } else {
                sb1.append('1');
            }
        }
        for (int i = 0; i<str.length(); i++) {
            if ((i & 1) == 0) {
                sb2.append('1');
            } else {
                sb2.append('0');
            }
        }

        int c1 = 0;
        int c2 = 0;
        for (int i=0; i<str.length(); i++) {
            if (sb1.charAt(i) != str.charAt(i)) {
                c1++;
            }
        }
        for (int i=0; i<str.length(); i++) {
            if (sb2.charAt(i) != str.charAt(i)) {
                c2++;
            }
        }
        out.println(Math.min(c1, c2));
        out.close();
    }

    static class Scanner {
        StringTokenizer st;
        BufferedReader br;

        public Scanner(InputStream s) {
            br = new BufferedReader(new InputStreamReader(s));
        }

        public Scanner(String s) {
            try {
                br = new BufferedReader(new FileReader(s));
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }
        }

        public String next() {
            while (st == null || !st.hasMoreTokens()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public String nextLine() {
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }

        public int[] nextIntArray(int n) {
            int[] arr = new int[n];
            for (int i = 0; i < n; i++) {
                arr[i] = nextInt();
            }
            return arr;
        }

        public double[] nextDoubleArray(int n) {
            double[] arr = new double[n];
            for (int i = 0; i < n; i++) {
                arr[i] = nextDouble();
            }
            return arr;
        }

        public long[] nextLongArray(int n) {
            long[] arr = new long[n];
            for (int i = 0; i < n; i++) {
                arr[i] = nextLong();
            }
            return arr;
        }

        public char[] nextCharArray(int n) {
            char[] arr = nextLine().trim().replaceAll("\\s", "").toCharArray();
            return arr;
        }

        public String[] nextStringArray(int n) {
            String[] arr = new String[n];
            for (int i = 0; i < n; i++) {
                arr[i] = next();
            }
            return arr;
        }
    }
}


Measured memory_bytes: 25300