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: Accepted)

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var (
	in = bufio.NewReader(os.Stdin)
)

func readln() string {
	buf := make([]byte, 0)
	for {
		line, prefix, err := in.ReadLine()
		if err != nil {
			panic(err)
		}
		buf = append(buf, line...)
		if prefix == false {
			break
		}
	}
	return string(buf)
}

func strSlice() []string {
	line := strings.Split(readln(), " ")
	return line
}

func intSlice() []int {
	line := strSlice()
	slice := make([]int, 0)
	for _, tmp := range line {
		val, err := strconv.Atoi(tmp)
		if err != nil {
			panic(err)
		}
		slice = append(slice, val)
	}
	return slice
}

func read() (T int, t []int) {
	line := intSlice()
	T = line[1]
	t = intSlice()
	return T, t
}

func minInt(a, b int) int {
	if a < b {
		return a
	}
	return b
}

func main() {
	T, t := read()

	x := uint64(T)
	for i := 0; i < len(t)-1; i++ {
		x += uint64(minInt(T, t[i+1]-t[i]))
	}
	fmt.Println(x)

}