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;

class Program
{
    static Scanner sc = new Scanner();
    static void Main(string[] args)
    {
        var S = sc.ReadStr();

        var ans0 = 0;
        var x = '1';
        for (int i = 0; i < S.Length; i++)
        {
            if (x == S[i])
            {
                ans0 += 1;
                x = S[i] == '1' ? '0' : '1';
            }
            else
            {
                x = S[i];
            }
        }
        var ans1 = 0;
        x = '0';
        for (int i = 0; i < S.Length; i++)
        {
            if (x == S[i])
            {
                ans1 += 1;
                x = S[i] == '0' ? '1' : '0';
            }
            else
            {
                x = S[i];
            }
        }

        var ans = Math.Min(ans0, ans1);
        Console.WriteLine(ans);
    }

    struct BC
    {
        public int B { get; set; }
        public int C { get; set; }
        public BC(int _B, int _C)
        {
            this.B = _B;
            this.C = _C;
        }
    }
}

class Modulo
{
    public readonly int MOD;
    private readonly List<int> Facs = new List<int>() { 1 };

    public Modulo(int mod = 1000000007) { this.MOD = mod; }
    public int Mul(int a, int b) { return (int)(Math.BigMul(a, b) % MOD); }
    public int Div(int a, int b) { return Mul(a, Pow(b, MOD - 2)); }
    public int Pow(int a, int n)
    {
        switch (n)
        {
            case 0:
                return 1;
            case 1:
                return a;
            default:
                int p1 = Pow(a, n / 2);
                int p2 = Mul(p1, p1);
                return ((n % 2) == 0) ? p2 : Mul(p2, a);
        }
    }
    public int Fac(int n)
    {
        for (int i = this.Facs.Count; i <= n; i++)
        {
            this.Facs.Add(Mul(i, Facs[i - 1]));
        }
        return this.Facs[n];
    }
    public int nPr(int n, int r) => Div(Fac(n), Fac(n - r));
    public int nCr(int n, int r)
    {
        if (n < r) return 0;
        if (n == r) return 1;
        int res = Fac(n);
        res = Div(res, Fac(r));
        res = Div(res, Fac(n - r));
        return res;
    }
}

class Debug
{
    public static void Dump<T>(T[,] array)
    {
        Console.WriteLine();
        var h = array.GetLength(0);
        var w = array.GetLength(1);

        for (int r = 0; r < h; r++)
        {
            for (int c = 0; c < w; c++)
            {
                Console.Write(array[r, c]);
                Console.Write(" ");
            }
            Console.WriteLine();
        }
        Console.WriteLine();
    }
    public static void Dump<T>(T[] array)
    {
        Console.WriteLine();
        Console.WriteLine(string.Join(" ", array));
        Console.WriteLine();
    }
}

class Scanner
{
    string[] S = new string[0];
    int Index = 0;
    char[] Separators = new char[] { ' ' };

    public string Next()
    {
        if (this.Index < this.S.Length) return this.S[this.Index++];
        var line = "";
        while (line == "") line = Console.ReadLine();
        this.S = line.Split(this.Separators, StringSplitOptions.RemoveEmptyEntries);
        if (this.S.Length == 0) return this.Next();
        this.Index = 0;
        return this.S[this.Index++];
    }

    public string ReadStr()
    {
        return this.Next();
    }

    public char ReadChar()
    {
        return this.Next()[0];
    }

    public int ReadInt()
    {
        return int.Parse(this.Next());
    }

    public uint ReadUInt()
    {
        return uint.Parse(this.Next());
    }

    public long ReadLong()
    {
        return long.Parse(this.Next());
    }

    public int[] ReadIntArray(int n)
    {
        var array = new int[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadInt();
        }
        return array;
    }

    public long[] ReadLongArray(int n)
    {
        var array = new long[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadLong();
        }
        return array;
    }

    public char[] ReadCharArray(int n)
    {
        var array = new char[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadChar();
        }
        return array;
    }

    public string[] ReadStrArray(int n)
    {
        var array = new string[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadStr();
        }
        return array;
    }
}

class HashMap<K, V> : Dictionary<K, V> where V : new()
{
    private V DefaltValue = new V();
    public HashMap() { }
    public HashMap(V defaultValue)
    {
        this.DefaltValue = new V();
    }
    new public V this[K i]
    {
        get
        {
            V v;
            return TryGetValue(i, out v) ? v : base[i] = this.DefaltValue;
        }
        set { base[i] = value; }
    }
}

class PriorityQueue<T> where T : IComparable<T>
{
    public readonly T[] Array;
    public readonly bool Greater;
    public int Count { get; private set; } = 0;

    public PriorityQueue(int capacity, bool greater = true)
    {
        this.Array = new T[capacity];
        this.Greater = greater;
    }

    public void Enqueue(T item)
    {
        this.Array[this.Count] = item;
        this.Count += 1;
        this.UpHeap();
    }

    public T Dequeue()
    {
        this.Swap(0, this.Count - 1);
        this.Count -= 1;
        this.DownHeap();

        return this.Array[this.Count];
    }

    public T Peek()
    {
        return this.Array[0];
    }

    private void UpHeap()
    {
        var n = this.Count - 1;
        while (n != 0)
        {
            var parent = (n - 1) / 2;

            if (this.Array[n].CompareTo(this.Array[parent]) > 0)
            {
                this.Swap(n, parent);
                n = parent;
            }
            else
            {
                break;
            }
        }
    }

    private void DownHeap()
    {
        var parent = 0;
        while (true)
        {
            var child = 2 * parent + 1;
            if (child > this.Count - 1) break;

            if (child < this.Count - 1 && this.Compare(this.Array[child], this.Array[child + 1]) < 0)
            {
                child += 1;
            }

            if (this.Compare(this.Array[parent], this.Array[child]) < 0)
            {
                this.Swap(parent, child);
                parent = child;
            }
            else
            {
                break;
            }
        }
    }

    private int Compare(T a, T b)
    {
        var c = a.CompareTo(b);
        if (!this.Greater)
        {
            c = -c;
        }
        return c;
    }

    private void Swap(int a, int b)
    {
        var tmp = this.Array[a];
        this.Array[a] = this.Array[b];
        this.Array[b] = tmp;
    }
}