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.Text;

public class ABC0124_A3v2 {
    private static void Main(string[] args) {
        string input = Console.ReadLine().Trim();
        ConvertInput(input);
    }

    private static void ConvertInput(string input) {
        var satisfyLists = GetSatisfyLists(input.Length);

        Solve(input, satisfyLists);
    }

    private static void Solve(string input, List<string> satisfyLists) {
        var countA = CountDifferentPairs(input, satisfyLists[0]);
        var countB = CountDifferentPairs(input, satisfyLists[1]);
        OutputAnswer(Math.Min(countA, countB).ToString());
    }

    private static void OutputAnswer(string text) => Console.WriteLine(text);

    private static List<string> GetSatisfyLists(int size) {
        var sb1 = new StringBuilder();
        var sb2 = new StringBuilder();

        for (int index = 0; index < size; index++) {
            if (index % 2 == 0) {
                sb1.Append("1");
                sb2.Append("0");

            } else {
                sb1.Append("0");
                sb2.Append("1");
            }
        }

        var satisfyLists = new List<string>() {
            sb1.ToString(),
            sb2.ToString(),
        };

        return satisfyLists;
    }

    private static int CountDifferentPairs(string a, string b) {
        var count = 0;

        for (var index = 0; index < a.Length; index++) {
            if (a[index] == b[index])
                count++;
        }

        return count;
    }
}