Problem:
Score : 200 points

Problem Statement

You are given nonnegative integers a and b (a ≤ b), and a positive integer x.
Among the integers between a and b, inclusive, how many are divisible by x?

Constraints

0 ≤ a ≤ b ≤ 10^{18}

1 ≤ x ≤ 10^{18}

Input

The input is given from Standard Input in the following format:

a b x

Output

Print the number of the integers between a and b, inclusive, that are divisible by x.

Sample Input 1

4 8 2

Sample Output 1

3

There are three integers between 4 and 8, inclusive, that are divisible by 2: 4, 6 and 8.

Sample Input 2

0 5 1

Sample Output 2

6

There are six integers between 0 and 5, inclusive, that are divisible by 1: 0, 1, 2, 3, 4 and 5.

Sample Input 3

9 9 2

Sample Output 3

0

There are no integer between 9 and 9, inclusive, that is divisible by 2.

Sample Input 4

1 1000000000000000000 3

Sample Output 4

333333333333333333

Watch out for integer overflows.

Solution in C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

using System;
using System.Collections.Generic;
using System.Linq;
using static System.Console;
using static System.Math;
using static ConsoleApplication.Utils;

namespace ConsoleApplication
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var abx = ReadUlongs().ToArray();
            var start = abx[2] * (ulong)Ceiling((double)abx[0] / abx[2]);
            var count = start < abx[1] ? (abx[1] - start) / abx[2] + 1 : 0;
            WriteLine(count);
        }
    }

    public static class Utils
    {
        #region Extensions for IO

        public static int ReadInt()
            => ReadLine().ToInt();

        public static IEnumerable<int> ReadInts()
            => ReadStrings().Select(int.Parse);

        public static long ReadLong()
            => ReadLine().ToLong();

        public static IEnumerable<long> ReadLongs()
            => ReadStrings().Select(long.Parse);

        public static ulong ReadUlong()
            => ReadLine().ToUlong();

        public static IEnumerable<ulong> ReadUlongs()
            => ReadStrings().Select(ulong.Parse);
            

        public static IEnumerable<string> ReadStrings()
            => ReadLine().Split(' ');

        public static IEnumerable<string> ReadLines(int n)
        {
            foreach (var i in Enumerable.Range(0, n))
            {
                yield return ReadLine();
            }
        }

        #endregion

        #region Extensions of int

        public static bool IsOdd(this int n)
            => !n.IsEven();

        public static bool IsEven(this int n)
            => n % 2 == 0;

        public static bool CanDivideWith(this int n, int divisor)
            => n % divisor == 0;

        #endregion

        #region Extensions of string

        public static int ToInt(this string str)
            => int.Parse(str);

        public static long ToLong(this string str)
            => long.Parse(str);

        public static ulong ToUlong(this string str)
            => ulong.Parse(str);

        public static double ToDouble(this string str)
            => double.Parse(str);

        public static bool IsNullOrEmpty(this string str)
            => string.IsNullOrEmpty(str);

        #endregion

        public static TSource Min<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
            where TKey : IComparable
            => source.OrderBy(keySelector).First();

        public static TSource Max<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
            where TKey : IComparable
            => source.OrderByDescending(keySelector).First();

        public static void ForEach<T>(this IEnumerable<T> source, Action<T> action)
        {
            foreach (var item in source)
            {
                action(item);
            }
        }

        public static IEnumerable<T> Peek<T>(this IEnumerable<T> source, Action<T> action)
        {
            foreach (var item in source)
            {
                action(item);
            }
            return source;
        }
    }
}