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: Wrong Answer)

using System;
using System.Linq;
using System.Collections.Generic;
using System.Text;

namespace Program
{
    public class ABC124C
    {
        static public void Main(string[] args)
        {
            var S = NextString;
            var list = new List<int>();
            var cnt = 0;
            var prev = S[0];
            foreach (var item in S.ToArray())
            {
                if (prev != item)
                {
                    list.Add(cnt);
                    cnt = 0;
                }
                ++cnt;
                prev = item;
            }
            list.Add(cnt);
            var ans1 = list.First() / 2;
            for (int i = 1; i < list.Count - 1; ++i)
            {
                ans1 += list[i] / 2;
                if (list[i] % 2 == 0) ++list[i + 1];
            }
            if (list.Count > 1) ans1 += list.Last() / 2;
            var ans2 = 0;
            for (int i = 0; i < list.Count - 1; ++i)
            {
                ans2 += list[i] / 2;
                if (list[i] % 2 == 0) ++list[i + 1];
            }
            if (list.Count > 1 || ans2 == 0) ans2 += list.Last() / 2;

            Console.WriteLine(Math.Min(ans2, ans1));
        }

        static class Console_
        {
            private static Queue<string> param = new Queue<string>();
            public static string NextString()
            {
                if (param.Count == 0) foreach (var item in Console.ReadLine().Split(' ')) param.Enqueue(item);
                return param.Dequeue();
            }
        }
        static int NextInt => int.Parse(Console_.NextString());
        static int[] NextIntArray(int N)
        {
            var ret = new int[N];
            for (int i = 0; i < N; ++i) ret[i] = NextInt;
            return ret;
        }
        static long NextLong => long.Parse(Console_.NextString());
        static long[] NextLongArray(int N)
        {
            var ret = new long[N];
            for (int i = 0; i < N; ++i) ret[i] = NextLong;
            return ret;
        }
        static string NextString => Console_.NextString();
        static void Rep(int count, Action<int> func)
        {
            for (int i = 0; i < count; ++i) func(i);
        }
        static void Rep(int count, Action func)
        {
            for (int i = 0; i < count; ++i) func();
        }
        static void Foreach<T>(IEnumerable<T> list, Action<T> func)
        {
            foreach (var item in list) func(item);
        }
        static void Foreach<T>(IEnumerable<T> list, Action<T, int> func)
        {
            int idx = -1;
            foreach (var item in list) func(item, ++idx);
        }
        static IEnumerable<long> Prime(long x)
        {
            if (x < 2) yield break;
            yield return 2;
            var halfx = x / 2;
            var table = new bool[halfx + 1];
            var max = (long)(Math.Sqrt(x) / 2);
            for (long i = 1; i <= max; ++i)
            {
                if (table[i]) continue;
                var add = 2 * i + 1;
                yield return add;
                for (long j = 2 * i * (i + 1); j <= halfx; j += add) table[j] = true;
            }
            for (long i = max + 1; i <= halfx; ++i) if (!table[i]) yield return 2 * i + 1;
        }
        static IEnumerable<long> Divisor(long x)
        {
            if (x < 1) yield break;
            var max = (long)Math.Sqrt(x);
            for (long i = 1; i < max; ++i)
            {
                if (x % i != 0) continue;
                yield return i;
                if (i != x / i) yield return x / i;
            }
        }
        static long GCD(long a, long b)
        {
            long tmpa = a, tmpb = b;
            while (tmpb > 0) { var tmp = tmpb; tmpb = tmpa % tmpb; tmpa = tmp; }
            return tmpa;
        }
    }
}