Problem:
Score : 300 points

Problem Statement

N tiles are arranged in a row from left to right. The initial color of each tile is represented by a string S of length N.

The i-th tile from the left is painted black if the i-th character of S is 0, and painted white if that character is 1.

You want to repaint some of the tiles black or white, so that any two adjacent tiles have different colors.

At least how many tiles need to be repainted to satisfy the condition?

Constraints

1 \leq |S| \leq 10^5

S_i is 0 or 1.

Input

Input is given from Standard Input in the following format:

S

Output

Print the minimum number of tiles that need to be repainted to satisfy the condition.

Sample Input 1

000

Sample Output 1

1

The condition can be satisfied by repainting the middle tile white.

Sample Input 2

10010010

Sample Output 2

3

Sample Input 3

0

Sample Output 3

0

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Diagnostics;

public class Program
{
    public static readonly int Mod = 1000000007;

    public static void Main(string[] args)
    {
        string s = Scanner.String;
        int evenBlackC = 0;
        int oddBlackC = 0;
        foreach (int i in Enumerable.Range(0, s.Length))
        {
            if(s[i] == '0')
            {
                if (i % 2 == 0)
                {
                    evenBlackC++;
                }
                else
                {
                    oddBlackC++;
                }
            }
        }

        int evenAll = (s.Length / 2) + ((s.Length % 2 == 1) ? 1 : 0);
        int oddAll = s.Length / 2;

        int evenWhite = evenAll - evenBlackC;
        int oddWhite = oddAll - oddBlackC;

        int evenB = evenWhite + oddBlackC;
        int oddB = oddWhite + evenBlackC;

        Console.WriteLine(Math.Min(evenB, oddB));
        Console.ReadLine();
    }
}

/// <summary>
/// スキャナ
/// </summary>
public static class Scanner
{
    public static char[] separator = new char[] { ' ' };

    private static IEnumerator<string> enumerator = Enumerator();
    private static IEnumerator<string> Enumerator()
    {
        while (true)
        {
            foreach (string s in Console.ReadLine().Split(separator))
            {
                yield return s;
            }
        }
    }

    public static IEnumerable<string> StringLine
    {
        get
        {
            return Console.ReadLine().Split(separator);
        }
    }
    public static IEnumerable<int> IntLine
    {
        get
        {
            return StringLine.Select(int.Parse);
        }
    }
    public static IEnumerable<long> LongLine
    {
        get
        {
            return StringLine.Select(long.Parse);
        }
    }
    public static IEnumerable<double> DoubleLine
    {
        get
        {
            return StringLine.Select(double.Parse);
        }
    }
    public static IEnumerable<char> CharLine
    {
        get
        {
            return StringLine.Select(char.Parse);
        }
    }

    public static string String
    {
        get
        {
            return enumerator.Next();
        }
    }
    public static int Int
    {
        get
        {
            return String.Int();
        }
    }
    public static long Long
    {
        get
        {
            return String.Long();
        }
    }
    public static double Double
    {
        get
        {
            return String.Double();
        }
    }
    public static char Char
    {
        get
        {
            return char.Parse(String);
        }
    }

    public static Tuple<int, int> IntTuple
    {
        get
        {
            return new Tuple<int, int>(Int, Int);
        }
    }

}

/// <summary>
/// Linq拡張
/// </summary>
public static class LinqExtension
{
    /// <summary>
    /// 二分探索
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="self"></param>
    /// <param name="solve"></param>
    /// <param name="min"></param>
    /// <param name="max"></param>
    /// <returns></returns>
    public static T LeftBinarySearch<T>(this IEnumerable<T> self, Func<T, bool> solve, T min, T max)
    {
        int index = self.LeftBinarySearchIndex(solve);

        return self.Get(index, min, max);
    }

    /// <summary>
    /// 二分探索
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="self"></param>
    /// <param name="solve"></param>
    /// <param name="min"></param>
    /// <param name="max"></param>
    /// <returns></returns>
    public static T RightBinarySearch<T>(this IEnumerable<T> self, Func<T, bool> solve, T min, T max)
    {
        int index = self.RightBinarySearchIndex(solve);

        return self.Get(index, min, max);
    }

    /// <summary>
    /// <para>二分探索</para>
    /// <para>渡されたラムダ式の条件を満たすものの中で、一番左側のもの(インデックスが小さい)</para>
    /// </summary>
    /// <typeparam name="T">IEnumerable</typeparam>
    /// <param name="solve"></param>
    /// <returns>一番左側のインデックス</returns>
    public static int LeftBinarySearchIndex<T>(this IEnumerable<T> self, Func<T, bool> solve)
    {
        int ng = -1;
        int ok = self.Count();

        while (Math.Abs(ok - ng) > 1)
        {
            int mid = (ok + ng) / 2;

            if (solve(self.ElementAt(mid)))
            {
                ok = mid;
            }
            else
            {
                ng = mid;
            }
        }

        return ok;
    }

    /// <summary>
    /// <para>二分探索</para>
    /// <para>渡されたラムダ式の条件を満たすものの中で、一番右側のもの(インデックスが大きい)</para>
    /// </summary>
    /// <typeparam name="T">IEnumerable</typeparam>
    /// <param name="solve"></param>
    /// <returns>一番右側のインデックス</returns>
    public static int RightBinarySearchIndex<T>(this IEnumerable<T> self, Func<T, bool> solve)
    {
        int ng = self.Count();
        int ok = -1;

        while (Math.Abs(ok - ng) > 1)
        {
            int mid = (ok + ng) / 2;

            if (solve(self.ElementAt(mid)))
            {
                ok = mid;
            }
            else
            {
                ng = mid;
            }
        }

        return ok;
    }

    /// <summary>
    /// OutOfRange対応
    /// </summary>
    /// <typeparam name="T">型T</typeparam>
    /// <param name="self">IEnumerable</param>
    /// <param name="index">対象のインデックス</param>
    /// <param name="min">0未満のインデックスの時</param>
    /// <param name="max">配列のサイズ以上の時</param>
    /// <returns>型Tの値</returns>
    public static T Get<T>(this IEnumerable<T> self, int index, T min, T max)
    {
        if (index < 0) return min;
        if (index >= self.Count()) return max;

        return self.ElementAt(index);
    }

    /// <summary>
    /// リストを埋める
    /// </summary>
    /// <typeparam name="T">型T</typeparam>
    /// <param name="self">リスト</param>
    /// <param name="func">(index) => T</param>
    /// <returns>リストの参照</returns>
    public static IList<T> Fill<T>(this IList<T> self, Func<int, T> func)
    {
        for (int i = 0; i < self.Count; i++)
        {
            self[i] = func(i);
        }

        return self;
    }

    /// <summary>
    /// リストを埋める
    /// </summary>
    /// <typeparam name="T">型T</typeparam>
    /// <param name="self">リスト</param>
    /// <param name="value">型Tの値</param>
    /// <returns>リストの参照</returns>
    public static IList<T> Fill<T>(this IList<T> self, T value)
    {
        for (int i = 0; i < self.Count; i++)
        {
            self[i] = value;
        }

        return self;
    }

    public static T Next<T>(this IEnumerator<T> self)
    {
        self.MoveNext();
        return self.Current;
    }

    public static void Write(this IEnumerable<bool> self)
    {
        StringBuilder sb = new StringBuilder();
        foreach(bool b in self)
        {
            sb.Append(b ? "□" : "■");
        }
        Console.WriteLine(sb.ToString());
    }

    public static U[] SelectArray<T, U>(this IEnumerable<T> self, Func<T, U> func)
    {
        U[] array = new U[self.Count()];

        int i = 0;
        foreach(T t in self)
        {
            array[i] = func(t);
            i++;
        }

        return array;
    }

    public static List<U> SelectList<T, U>(this IEnumerable<T> self, Func<T, U> func)
    {
        List<U> list = new List<U>(self.Count());

        foreach (T t in self)
        {
            list.Add(func(t));
        }

        return list;
    }
}

/// <summary>
/// Int拡張
/// </summary>
public static class IntExtensiton
{
    /// <summary>
    /// 階上
    /// </summary>
    /// <param name="self"></param>
    /// <returns></returns>
    public static long Factorial(this int self)
    {
        if (self == 1) return 1L;
        return self * (self - 1).Factorial();
    }

    /// <summary>
    /// 階上
    /// </summary>
    /// <param name="self"></param>
    /// <returns></returns>
    public static long Factorial(this int self, int c)
    {
        if (c == 0 || self == 1) return 1L;
        return self * Factorial(self - 1, c - 1);
    }

    public static int BitCount(this int bit, int digit = 32)
    {
        int count = 0;
        for (int i = (digit - 1); i >= 0; i--)
        {
            if (bit.GetBit(i)) count++;
        }
        return count;
    }

    public static bool GetBit(this int bit, int digit)
    {
        int mask = (1 << digit);
        return (bit & mask) == mask;
    }

    public static int SetBit(this int bit, int digit)
    {
        return (bit | (1 << digit));
    }

    public static string BitToString(this int bit, int digit = 32)
    {
        StringBuilder sb = new StringBuilder();

        for (int i = (digit - 1); i >= 0; i--)
        {
            sb.Append(bit.GetBit(i) ? "1" : "0");
        }

        return sb.ToString();
    }

    public static bool IsDivisor(this int self, int target)
    {
        return self % target == 0;
    }

    public static IEnumerable<int> Divisors(this int self)
    {
        for (int i = 1; i * i <= self; i++)
        {
            if (self % i == 0)
            {
                yield return i;
                if (i * i != self) yield return self / i;
            }
        }
    }

    public static bool NeighboringBit(this int self, int digit = 32)
    {
        if (digit < 2) return false;
        return Enumerable.Range(0, digit - 1).Where(j => self.GetBit(j) && self.GetBit(j + 1)).Any();
    }
}

/// <summary>
/// Long拡張
/// </summary>
public static class LongExtensition
{
    public static long BitCount(this long bit, int digit = 64)
    {
        long count = 0;
        for (int i = (digit - 1); i >= 0; i--)
        {
            if (bit.GetBit(i)) count++;
        }
        return count;
    }

    public static bool GetBit(this long bit, int digit)
    {
        long mask = (1L << digit);
        return (bit & mask) == mask;
    }

    public static long SetBit(this long bit, int digit)
    {
        return bit | (1L << digit);
    }

    public static string BitToString(this long bit, int digit = 64)
    {
        StringBuilder sb = new StringBuilder();

        for (int i = (digit - 1); i >= 0; i--)
        {
            sb.Append(bit.GetBit(i) ? "1" : "0");
        }

        return sb.ToString();
    }

    public static bool IsDivisor(this long self, int target)
    {
        return self % target == 0;
    }
}

/// <summary>
/// String拡張
/// </summary>
public static class StringExtensiton
{
    public static void Print(this string str) => Console.WriteLine(str);
    public static int Int(this string str) => int.Parse(str);
    public static long Long(this string str) => long.Parse(str);
    public static double Double(this string str) => double.Parse(str);
}

/// <summary>
/// 便利関数
/// </summary>
public static class F
{
    public static void ChMax<T>(ref T x, T y) where T : IComparable
    {
        if (x.CompareTo(y) < 0)
        {
            x = y;
        }
    }

    public static void ChMin<T>(ref T x, T y) where T : IComparable
    {
        if (x.CompareTo(y) > 0)
        {
            x = y;
        }
    }

    public static void Swap<T>(ref T x, ref T y)
    {
        T tmp = x;

        x = y;
        y = tmp;
    }
}