Problem:
Score : 300 points

Problem Statement

In a public bath, there is a shower which emits water for T seconds when the switch is pushed.

If the switch is pushed when the shower is already emitting water, from that moment it will be emitting water for T seconds.
Note that it does not mean that the shower emits water for T additional seconds.

N people will push the switch while passing by the shower.
The i-th person will push the switch t_i seconds after the first person pushes it.

How long will the shower emit water in total?

Constraints

1 ≤ N ≤ 200,000

1 ≤ T ≤ 10^9

0 = t_1 < t_2 < t_3 < , ..., < t_{N-1} < t_N ≤ 10^9

T and each t_i are integers.

Input

Input is given from Standard Input in the following format:

N T
t_1 t_2 ... t_N

Output

Assume that the shower will emit water for a total of X seconds. Print X.

Sample Input 1

2 4
0 3

Sample Output 1

7

Three seconds after the first person pushes the water, the switch is pushed again and the shower emits water for four more seconds, for a total of seven seconds.

Sample Input 2

2 4
0 5

Sample Output 2

8

One second after the shower stops emission of water triggered by the first person, the switch is pushed again.

Sample Input 3

4 1000000000
0 1000 1000000 1000000000

Sample Output 3

2000000000

Sample Input 4

1 1
0

Sample Output 4

1

Sample Input 5

9 10
0 3 5 7 100 110 200 300 311

Sample Output 5

67

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

using System;

namespace Sentou
{
    class Program
    {
        static long[] ToInt(string[] str)
        {
            long len = str.Length;
            long[] intAry = new long[len];
            for (long i = 0; i < len; i++)
            {
                intAry[i] = long.Parse(str[i]);
            }
            return intAry;
        }
        static void Main(string[] args)
        {
            long[] NT = ToInt(Console.ReadLine().Split(' '));
            long[] T = ToInt(Console.ReadLine().Split(' '));
            long sum = 0, start = 0, end = 0;

            for (long i = 0; i < T.Length; i++)
            {
                if(NT[1] < T[i])
                {
                    sum += end - start;
                    start = T[i];
                    end = T[i] + NT[1];
                    
                }else
                {
                end = T[i] + NT[1];
                }
            }
            sum += end - start;
            Console.WriteLine(sum);
        }
    }
}