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.IO;
using System.Runtime.Remoting.Contexts;
using Microsoft.Win32;

namespace Contest
{
    class Program
    {
        private Scanner sc;

        public void Solve()
        {
            sc = new Scanner();
            string S = sc.Next();

            int a = 0;
            int b = 0;
            for (int i = 0; i < S.Length; i++)
            {
                if (i % 2 == S[i] - '0')
                {
                    a++;
                }
                else
                {
                    b++;
                }
            }
            Console.WriteLine(Math.Min(a,b));
        }

        static void Main() => new Program().Solve();
    }
}

struct ModInt
{
    public long Num;
    public const long Mod = (long)1e9 + 7;
    public ModInt(long n) { Num = n % Mod; }

    public override string ToString() { return Num.ToString(); }
    public override bool Equals(object b) { return Num.Equals(b); }

    public static ModInt operator +(ModInt a, ModInt b)
    {
        return (a.Num + b.Num) % Mod;
    }
    public static ModInt operator -(ModInt a, ModInt b)
    {
        return (a.Num - b.Num) % Mod;
    }
    public static ModInt operator *(ModInt a, ModInt b)
    {
        return (a.Num * b.Num) % Mod;
    }
    public static ModInt operator %(ModInt a, long b)
    {
        return ((a.Num % b) + Mod) % Mod;
    }
    public static ModInt operator /(ModInt a, ModInt b)
    {
        return a * ModInt.Pow(b, Mod - 2);
    }

    public static ModInt Pow(ModInt a, long b)
    {
        if (b == 0) return 1;
        return (b % 2 == 1 ? a : 1) * Pow(a * a, b / 2);
    }

    public static implicit operator ModInt(long n) { return new ModInt(n); }


}

class Scanner
{
    public Scanner()
    {
        _stream = new StreamReader(Console.OpenStandardInput());
        _pos = 0;
        _line = new string[0];
        _separator = ' ';
    }
    private readonly char _separator;
    private readonly StreamReader _stream;
    private int _pos;
    private string[] _line;
    #region get a element
    public string Next()
    {
        if (_pos >= _line.Length)
        {
            _line = _stream.ReadLine().Split(_separator);
            _pos = 0;
        }
        return _line[_pos++];
    }
    public int NextInt()
    {
        return int.Parse(Next());
    }
    public long NextLong()
    {
        return long.Parse(Next());
    }
    public double NextDouble()
    {
        return double.Parse(Next());
    }
    #endregion
    #region convert array
    private int[] ToIntArray(string[] array)
    {
        var result = new int[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = int.Parse(array[i]);
        }

        return result;
    }
    private long[] ToLongArray(string[] array)
    {
        var result = new long[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = long.Parse(array[i]);
        }

        return result;
    }
    private double[] ToDoubleArray(string[] array)
    {
        var result = new double[array.Length];
        for (int i = 0; i < array.Length; i++)
        {
            result[i] = double.Parse(array[i]);
        }

        return result;
    }
    #endregion
    #region get array
    public string[] Array()
    {
        if (_pos >= _line.Length)
            _line = _stream.ReadLine().Split(_separator);
        _pos = _line.Length;
        return _line;
    }
    public int[] IntArray()
    {
        return ToIntArray(Array());
    }
    public long[] LongArray()
    {
        return ToLongArray(Array());
    }
    public double[] DoubleArray()
    {
        return ToDoubleArray(Array());
    }
    #endregion
}