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 Haskell (GHC 7.10.3) (judge verdict: Accepted)

main :: IO ()
main = do
    [n, a] <- (map read . words) <$> getLine
    ts <- (map read . words) <$> getLine
    print $ solve (n, a) ts

-- |
--
-- >>> solve (2, 4) [0, 3]
-- 7
--
-- >>> solve (2, 4) [0, 5]
-- 8
--
-- >>> solve (9, 10) [0,3,5,7,100,110,200,300,311]
-- 67
--
-- >>> solve (4, 1000000000) [0,1000,1000000,1000000000]
-- 2000000000
solve :: (Int, Int) -> [Int] -> Int
solve (n, a) ts = n * a - snd (foldl f (-a, 0) ts)
  where f (x, acc) y = (y, acc + max 0 (x + a - y))