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 Go (1.6) (judge verdict: Wrong Answer)

package main

import (
	"fmt"
	"sync"
)

func main() {
	var n, t uint64
	fmt.Scanf("%d %d", &n, &t)
	ti := make([]uint64, n)
	for i := uint64(0); i < n; i++ {
		fmt.Scan(&ti[i])
	}
	sum := uint64(t)
	limit := make(chan struct{}, 100000)
	wg := &sync.WaitGroup{}
	for i := uint64(1); i < n; i++ {
		limit <- struct{}{}
		wg.Add(1)
		go func(a, b uint64) {
			defer wg.Done()
			tmp := a - b
			if t < tmp {
				sum += t
			} else {
				sum += tmp
			}
			<-limit
		}(ti[i], ti[i-1])
	}
	wg.Wait()
	fmt.Println(sum)
}