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.Text;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using static System.Console;
using static System.Math;

namespace AtCoder
{
    public class Program
    {
        public static void Main(string[] args)
        {
            new Program().Solve(new ConsoleInput(Console.In, ' '));
        }

        public void Solve(ConsoleInput cin)
        {
            var N = cin.ReadLine;
            var charArray = N.ToCharArray();
            var charArray2 = N.ToCharArray();
            long counter = charArray.Count();
            long result1 = 0;
            long result2 = 0;
            if (charArray2[0] == '0')
            {
                charArray2[0] = '1';
                ++result2;
            }
            else
            {
                charArray2[0] = '0';
                ++result2;
            }

            for (int i = 0; i < counter-1; i++)
            {
                if (charArray[i] == charArray[i+1])
                {
                    if (charArray[i+1] == '0')
                    {
                        charArray[i + 1] = '1';
                    }
                    else
                    {
                        charArray[i + 1] = '0';
                    }
                    ++result1;
                }

                if (charArray2[i] == charArray2[i + 1])
                {
                    if (charArray2[i + 1] == '0')
                    {
                        charArray2[i + 1] = '1';
                    }
                    else
                    {
                        charArray2[i + 1] = '0';
                    }
                    ++result2;
                }
            }
            if (result1 > result2)
            {
                Console.Write(result2.ToString());
            }
            else
            {
                Console.Write(result1.ToString());
            }
        }
    }
    public class deci
    {
        public bool IsFinish { get; set; } = false;
        public int Len { get; set; }
    }
    public class ConsoleInput
    {
        private readonly System.IO.TextReader _stream;
        private char _separator = ' ';
        private Queue<string> inputStream;
        public ConsoleInput(System.IO.TextReader stream, char separator = ' ')
        {
            this._separator = separator;
            this._stream = stream;
            inputStream = new Queue<string>();
        }
        public string Read
        {
            get
            {
                if (inputStream.Count != 0) return inputStream.Dequeue();
                string[] tmp = _stream.ReadLine().Split(_separator);
                for (int i = 0; i < tmp.Length; ++i)
                    inputStream.Enqueue(tmp[i]);
                return inputStream.Dequeue();
            }
        }
        public string ReadLine { get { return _stream.ReadLine(); } }
        public int ReadInt { get { return int.Parse(Read); } }
        public long ReadLong { get { return long.Parse(Read); } }
        public double ReadDouble { get { return double.Parse(Read); } }
        public string[] ReadStrArray(long N) { var ret = new string[N]; for (long i = 0; i < N; ++i) ret[i] = Read; return ret; }
        public int[] ReadIntArray(long N) { var ret = new int[N]; for (long i = 0; i < N; ++i) ret[i] = ReadInt; return ret; }
        public long[] ReadLongArray(long N) { var ret = new long[N]; for (long i = 0; i < N; ++i) ret[i] = ReadLong; return ret; }
    }
}