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 static System.Console;
using static System.Math;

namespace atcoder
{
    class Program
    {
        static void Main(string[] args)
        {
            do
            {
                new Program().Solve(new ConsoleInput(In));
            } while (args.Length > 0 && args[0] == "Debug");
        }

        public void Solve(ConsoleInput input)
        {
            var s = input.Read;
            var c0 = 0;
            var c1 = 0;

            for (var i = 0; i < s.Length; i++)
            {
                if (s[i] == new char[] { '0', '1' }[i % 2]) c0++;
            }
            for (var i = 0; i < s.Length; i++)
            {
                if (s[i] == new char[] { '1', '0' }[i % 2]) c1++;
            }
            WriteLine($"{Min(c0, c1)}");

        }
    }

    class Math
    {
        public static int Min(params int[] a)
        {
            if (a.Length == 0) return 0;

            var min = a[0];
            for (var i = 1; i < a.Length; i++)
            {
                if (min > a[i]) min = a[i];
            }
            return min;
        }
        public static long Min(params long[] a)
        {
            if (a.Length == 0) return 0;

            var min = a[0];
            for (var i = 1; i < a.Length; i++)
            {
                if (min > a[i]) min = a[i];
            }
            return min;
        }

        public static int Max(params int[] a)
        {
            if (a.Length == 0) return 0;

            var max = a[0];
            for (var i = 1; i < a.Length; i++)
            {
                if (max < a[i]) max = a[i];
            }
            return max;
        }
        public static long Max(params long[] a)
        {
            if (a.Length == 0) return 0;

            var max = a[0];
            for (var i = 1; i < a.Length; i++)
            {
                if (max < a[i]) max = a[i];
            }
            return max;
        }

        public static int Lcm(int a, int b)
        {
            return a * b / Gcd(a, b);
        }

        public static int Gcd(int a, int b)
        {
            if (a < b)
                return Gcd(b, a);
            while (b != 0)
            {
                var remainder = a % b;
                a = b;
                b = remainder;
            }
            return a;
        }
    }

    class UnionFind
    {
        private int[] parent;
        private int[] rank;
        private long[] size;

        public UnionFind(int count)
        {
            parent = new int[count];
            rank = new int[count];
            size = new long[count];
            for (var i = 0; i < count; i++)
            {
                parent[i] = i;
                rank[i] = 0;
                size[i] = 1;
            }
        }

        public int Root(int x)
        {
            if (parent[x] != x)
            {
                parent[x] = Root(parent[x]);
            }
            return parent[x];
        }

        public void Unite(int a, int b)
        {
            var ra = Root(a);
            var rb = Root(b);

            if (Size(ra) < Size(rb))
            {
                parent[ra] = rb;
                size[rb] += size[ra];
            }
            else
            {
                parent[rb] = ra;
                size[ra] += size[rb];
            }
            if (rank[ra] == rank[rb]) rank[ra]++;
        }

        public bool Same(int a, int b)
        {
            return Root(a) == Root(b);
        }

        public long Size(int x)
        {
            return size[Root(x)];
        }
    }

    class ConsoleInput
    {
        private readonly System.IO.TextReader _stream;
        private readonly char _separator;
        private Queue<string> inputStream;
        public ConsoleInput(System.IO.TextReader stream, char separator = ' ')
        {
            _separator = separator;
            _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; }
    }
}