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 Rust (1.15.1) (judge verdict: Accepted)

#![allow(dead_code, unused_imports, non_snake_case)]

use std::io::*;
use std::cmp::{max, min, Ordering};
use std::collections::*;
use std::str::FromStr;

const MOD: i32 = 1_000_000_007;

mod util {
    use std::fmt::Debug;
    use std::io::{stdin, stdout, BufWriter, StdoutLock};
    use std::str::FromStr;

    #[allow(dead_code)]
    pub fn line() -> String {
        let mut line: String = String::new();
        stdin().read_line(&mut line).unwrap();
        line.trim().to_string()
    }

    #[allow(dead_code)]
    pub fn chars() -> Vec<char> {
        line().chars().collect()
    }

    #[allow(dead_code)]
    pub fn gets<T: FromStr>() -> Vec<T>
        where
            <T as FromStr>::Err: Debug,
    {
        let mut line: String = String::new();
        stdin().read_line(&mut line).unwrap();
        line.split_whitespace()
            .map(|t| t.parse().unwrap())
            .collect()
    }

    #[allow(dead_code)]
    pub fn with_bufwriter<F: FnOnce(BufWriter<StdoutLock>) -> ()>(f: F) {
        let out = stdout();
        let writer = BufWriter::new(out.lock());
        f(writer)
    }
}

macro_rules! max {($x:expr)=>{$x};($x:expr,$($xs:tt)+)=>{max($x,max!($($xs)+))};}
macro_rules! min {($x:expr)=>{$x};($x:expr,$($xs:tt)+)=>{min($x,min!($($xs)+))};}
macro_rules! get {($t:ty)=>{{let mut line:String=String::new();stdin().read_line(&mut line).unwrap();line.trim().parse::<$t>().unwrap()}};($($t:ty),*)=>{{let mut line:String=String::new();stdin().read_line(&mut line).unwrap();let mut iter=line.split_whitespace();($(iter.next().unwrap().parse::<$t>().unwrap(),)*)}};($t:ty;$n:expr)=>{(0..$n).map(|_|get!($t)).collect::<Vec<_>>()};($($t:ty),*;$n:expr)=>{(0..$n).map(|_|get!($($t),*)).collect::<Vec<_>>()};($t:ty;;)=>{{let mut line:String=String::new();stdin().read_line(&mut line).unwrap();line.split_whitespace().map(|t|t.parse::<$t>().unwrap()).collect::<Vec<_>>()}};($t:ty;;$n:expr)=>{(0..$n).map(|_|get!($t;;)).collect::<Vec<_>>()};}
macro_rules! debug {($($a:expr),*)=>{eprintln!(concat!($(stringify!($a),"={:?},"),*),$($a),*);}}

fn main() {
    let (n, t) = get!(usize, usize);
    let ts = util::gets::<usize>();
    let mut ans = t;
    for i in 1..n {
        ans += min(t, ts[i] - ts[i - 1]);
    }
    println!("{}", ans);
}