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++14 (GCC 5.4.1) (judge verdict: Accepted)

/*************************************************************************
    > File Name: test.cpp
    > Author: Akira 
    > Mail: qaq.febr2.qaq@gmail.com 
 ************************************************************************/

#include<bits/stdc++.h>
typedef long long LL;
typedef unsigned long long ULL;
typedef long double LD;
#define MST(a,b) memset(a,b,sizeof(a))
#define CLR(a) MST(a,0)
#define Sqr(a) ((a)*(a))
using namespace std;

#define MaxN 200001
#define MaxM MaxN*10
#define INF 0x3f3f3f3f
#define PI 3.1415926535897932384626
const int mod = 1E9+7;
const double eps = 1e-6;
#define bug cout<<88888888<<endl;
#define debug(x) cout << #x" = " << x << endl;
int N;
LL T;
LL t[MaxN];
void solve()
{
    LL ans = 0;
    LL s = 0, e = 0;
    for(int i=1;i<=N;i++)
    {
        if(t[i]>e) 
        {
            ans += (e-s);
            s = t[i]; 
            e = t[i]+T;
        }
        else e = t[i]+T;
    }
    ans += e-s;
    printf("%lld\n", ans);
}
int main()
{
    //std::ios::sync_with_stdio(false);
    scanf("%d%lld",&N, &T);
    for(int i=1;i<=N;i++) scanf("%lld", &t[i]);
    solve();
    //system("pause");
}