import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        sc.close();

        int[] candidates = new int[2];
        for (int i = 0; i < s.length(); i++) {
            if (i % 2 == 0) {
                if (s.charAt(i) == '0') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            } else {
                if (s.charAt(i) == '1') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            }
        }
        int ans = Math.min(candidates[0], candidates[1]);
        System.out.println(ans);
    }

} import java.util.Scanner; Scanner java.util public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        sc.close();

        int[] candidates = new int[2];
        for (int i = 0; i < s.length(); i++) {
            if (i % 2 == 0) {
                if (s.charAt(i) == '0') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            } else {
                if (s.charAt(i) == '1') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            }
        }
        int ans = Math.min(candidates[0], candidates[1]);
        System.out.println(ans);
    }

} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        sc.close();

        int[] candidates = new int[2];
        for (int i = 0; i < s.length(); i++) {
            if (i % 2 == 0) {
                if (s.charAt(i) == '0') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            } else {
                if (s.charAt(i) == '1') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            }
        }
        int ans = Math.min(candidates[0], candidates[1]);
        System.out.println(ans);
    } main {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        sc.close();

        int[] candidates = new int[2];
        for (int i = 0; i < s.length(); i++) {
            if (i % 2 == 0) {
                if (s.charAt(i) == '0') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            } else {
                if (s.charAt(i) == '1') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            }
        }
        int ans = Math.min(candidates[0], candidates[1]);
        System.out.println(ans);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) String s = sc.next(); s sc.next() sc.next sc sc.close() sc.close sc int[] candidates = new int[2]; candidates new int[2] 2 for (int i = 0; i < s.length(); i++) {
            if (i % 2 == 0) {
                if (s.charAt(i) == '0') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            } else {
                if (s.charAt(i) == '1') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            }
        } int i = 0; int i = 0; i 0 i < s.length() i s.length() s.length s i++ i++ i {
            if (i % 2 == 0) {
                if (s.charAt(i) == '0') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            } else {
                if (s.charAt(i) == '1') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            }
        } {
            if (i % 2 == 0) {
                if (s.charAt(i) == '0') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            } else {
                if (s.charAt(i) == '1') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            }
        } if (i % 2 == 0) {
                if (s.charAt(i) == '0') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            } else {
                if (s.charAt(i) == '1') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            } i % 2 == 0 i % 2 i 2 0 {
                if (s.charAt(i) == '0') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            } if (s.charAt(i) == '0') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                } s.charAt(i) == '0' s.charAt(i) s.charAt s i '0' {
                    candidates[0]++;
                } candidates[0]++ candidates[0] candidates 0 {
                    candidates[1]++;
                } candidates[1]++ candidates[1] candidates 1 {
                if (s.charAt(i) == '1') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            } if (s.charAt(i) == '1') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                } s.charAt(i) == '1' s.charAt(i) s.charAt s i '1' {
                    candidates[0]++;
                } candidates[0]++ candidates[0] candidates 0 {
                    candidates[1]++;
                } candidates[1]++ candidates[1] candidates 1 int ans = Math.min(candidates[0], candidates[1]); ans Math.min(candidates[0], candidates[1]) Math.min Math candidates[0] candidates 0 candidates[1] candidates 1 System.out.println(ans) System.out.println System.out System System.out ans String[] args args public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        sc.close();

        int[] candidates = new int[2];
        for (int i = 0; i < s.length(); i++) {
            if (i % 2 == 0) {
                if (s.charAt(i) == '0') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            } else {
                if (s.charAt(i) == '1') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            }
        }
        int ans = Math.min(candidates[0], candidates[1]);
        System.out.println(ans);
    }

} public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        sc.close();

        int[] candidates = new int[2];
        for (int i = 0; i < s.length(); i++) {
            if (i % 2 == 0) {
                if (s.charAt(i) == '0') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            } else {
                if (s.charAt(i) == '1') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            }
        }
        int ans = Math.min(candidates[0], candidates[1]);
        System.out.println(ans);
    }

} Main
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        sc.close();

        int[] candidates = new int[2];
        for (int i = 0; i < s.length(); i++) {
            if (i % 2 == 0) {
                if (s.charAt(i) == '0') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            } else {
                if (s.charAt(i) == '1') {
                    candidates[0]++;
                } else {
                    candidates[1]++;
                }
            }
        }
        int ans = Math.min(candidates[0], candidates[1]);
        System.out.println(ans);
    }

}
