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.Linq;
using System.Collections.Generic;

namespace Contest
{
    class Program
    {
        static void Main(string[] args)
        {
            ABC124.C();
        }

        class ABC124
        {
            public static void A()
            {
                var A = Read.Int();
                var B = Read.Int();
                Console.WriteLine(Math.Max(Math.Max(A, B) * 2 - 1, A + B));
            }
            public static void B()
            {
                var N = Read.Int();
                var H = Read.IntN();
                int max = 0;
                int count = 0;
                for(int i = 0; i < N; ++i)
                {
                    if (max <= H[i]) { count++; }
                    max = Math.Max(max, H[i]);
                }
                Console.WriteLine(count);
            }
            public static void C()
            {
                var S = Read.Str();
                int count0 = 0, count1 = 0;
                for(int i = 0; i < S.Length; ++i)
                {
                    if (i % 2 == 0 ^ S[i] == '0')
                    {
                        count0++;
                    }
                    else
                    {
                        count1++;
                    }
                }
                Console.WriteLine(Math.Min(count0, count1));
            }
            public static void D()
            {
                var N = Read.Int();
                var K = Read.Int();
                var S = Read.Str();
                List<int> stand = new List<int>();
                List<int> reverse = new List<int>();
                int s = 0, r = 0;
                bool prevR = true;
                foreach(var c in S)
                {
                    if (prevR)
                    {
                        if (c == '1')
                        {
                            r++;
                        }
                        else
                        {
                            reverse.Add(r);
                            r = 0; s = 1;
                            prevR = false;
                        }
                    }
                    else
                    {
                        if (c == '0')
                        {
                            s++;
                        }
                        else
                        {
                            stand.Add(s);
                            s = 0;r = 1;
                            prevR = true;
                        }
                    }
                }
                if (prevR) { reverse.Add(r); } else { stand.Add(s);reverse.Add(0); }
                if (stand.Count <= K)
                {
                    Console.WriteLine(N);return;
                }
                int count = stand.Take(K).Sum() + reverse.Take(K + 1).Sum();
                int max = count;
                for(int i = K; i < stand.Count; ++i)
                {
                    count += reverse[i + 1] + stand[i];
                    count -= reverse[i - K] + stand[i - K];
                    max = Math.Max(max, count);
                }
                Console.WriteLine(max);
            }
        }

        class Util
        {
        }

        static class Read
        {
            static Queue<string> queue = new Queue<string>();
            static void Enqueue() { var ss = Console.ReadLine().Split(' '); foreach (var s in ss) { queue.Enqueue(s); } }
            public static int Int() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt32(queue.Dequeue()); }
            public static long Long() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt64(queue.Dequeue()); }
            public static string Str() { if (queue.Count == 0) { Enqueue(); } return queue.Dequeue(); }
            public static int[] IntN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt32(s)).ToArray(); }
            public static long[] LongN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt64(s)).ToArray(); }
            public static string[] StrN() { return Console.ReadLine().Split(' '); }
            public static Tuple<int, int>[] TupleInt(int n) { List<Tuple<int, int>> list = new List<Tuple<int, int>>(); for (int i = 0; i < n; ++i) { var ab = Read.IntN(); list.Add(new Tuple<int, int>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<long, long>[] TupleLong(int n) { List<Tuple<long, long>> list = new List<Tuple<long, long>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<long, long>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<string, string>[] TupleStr(int n) { List<Tuple<string, string>> list = new List<Tuple<string, string>>(); for (int i = 0; i < n; ++i) { var ab = Read.StrN(); list.Add(new Tuple<string, string>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<double, double>[] TupleDouble(int n) { List<Tuple<double, double>> list = new List<Tuple<double, double>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<double, double>(ab[0], ab[1])); } return list.ToArray(); }
            public static long[,] LongMatrix(int r, int c)
            {
                var mat = new long[r, c];
                for (int i = 0; i < r; ++i)
                {
                    var x = Read.LongN();
                    for (int j = 0; j < c; ++j)
                    {
                        mat[i, j] = x[j];
                    }
                }
                return mat;
            }
            public static Dictionary<int, HashSet<int>> AdjacencyList(int N, int M)
            {
                Dictionary<int, HashSet<int>> dict = new Dictionary<int, HashSet<int>>();
                for (int i = 0; i <= N; ++i)
                {
                    dict.Add(i, new HashSet<int>());
                }
                for (int i = 0; i < M; ++i)
                {
                    var ab = Read.IntN();
                    dict[ab[0]].Add(ab[1]);
                    dict[ab[1]].Add(ab[0]);
                }
                return dict;
            }
        }
    }
}