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: Time Limit Exceeded)

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

namespace ABC124C2
{
    class Program
    {
        static void Main(string[] args)
        {
            // 文字列の入力
            string S = Console.ReadLine();
            int sn = S.Length;
            string[] bit = new string[sn]; //Ｓの長さ分の配列を作る
            for (int i = 0; i < sn; i++)
            {
                bit[i] = S.Substring(i, 1);//配列にそれぞれの桁を記録する
            }
            string s0 = "";//目標となる文字列 s0
                string s1 = "";//目標となる文字列 s1
            if (sn%2 == 1){//文字列の長さが奇数のとき
                s0 = "0";
                s1 = "1";
                for (int i = 0; i < sn / 2; i++)
                {
                        s0 += "10";
                        s1 += "01";
                }
                int ans_0 = 0;//文字列s0との違いを求める
                int ans_1 = 0;//文字列s1との違いを求める
                for (int i = 0; i < sn; i++)
                {
                    if (bit[i] != s0.Substring(i, 1))
                    {
                        ans_0++;
                    }
                    if (bit[i] != s1.Substring(i, 1))
                    {
                        ans_1++;
                    }
                }
                Console.WriteLine(Math.Min(ans_0, ans_1));
            }
            else //文字列の長さが偶数のとき
            {
                for (int i = 0; i < sn/2; i++)
                {
                    s0 += "01";
                    s1 += "10";
                }
                int ans_0 = 0;//文字列s0との違い
                int ans_1 = 0;//文字列s1との違い
                for (int i = 0; i < sn; i++)
                {
                    if (bit[i] != s0.Substring(i, 1))
                    {
                        ans_0++;
                    }
                    if (bit[i] != s1.Substring(i, 1))
                    {
                        ans_1++;
                    }
                }
                Console.WriteLine(Math.Min(ans_0, ans_1));
            }
        }
    }
}