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;

public static partial class Program
{
    public static void Main()
    {
        var s = CRL();
        //01010101... => 0
        //10101010... => 1
        var count = new long[2];
        for (int i = 0; i < s.Length; i++)
        {
            if (i.IsEven())
            {
                if (s[i] == '0')
                {
                    count[1]++;
                }
                else
                {
                    count[0]++;
                }
            }
            else
            {
                if (s[i] == '1')
                {
                    count[1]++;
                }
                else
                {
                    count[0]++;
                }
            }
        }

        CWL(count.Min());
    }
}

public static partial class Program
{
    //テンプレここから
    private static long ToInt(this string str) => long.Parse(str);
    private static long[] ToInt(this IEnumerable<string> strs) => strs.Select(x => x.ToInt()).ToArray();
    public static long[] ArrayLongs(this string str, char opr = ' ') => str.Split(opr).ToInt();
    public static string CRL() => Console.ReadLine();
    public static void CWL(object obj) => Console.WriteLine(obj);
    public static long Diff(long a, long b) => Math.Abs(Math.Max(a, b) - Math.Min(a, b));
    public static long Sign(this long i) => Math.Sign(i);
    public static bool IsEven(this long i) => i % 2 == 0;
    public static bool IsEven(this int i) => i % 2 == 0;
    public static long[] dx = new long[] { 0, -1, 0, 1 };
    public static long[] dy = new long[] { -1, 0, 1, 0 };
    public static long mod = 1000000007;
    public static string alphabet = "abcdefghijklmnopqrstuvwxyz";
    public static long ToLong(this char c) => c.ToString().ToInt();
    public static long[] Sort(this long[] array)
    {
        var list = array.ToList();
        list.Sort();
        return list.ToArray();
    }
    public static long GCD(long a, long b) => b == 0 ? a : GCD(b, a % b);
    public static long LCM(long a, long b) => a * b / GCD(a, b);
    //テンプレここまで
}