Problem:
Max Score: 350 Points

Problem Statement

There are N buildings along the line. The i-th building from the left is colored in color i, and its height is currently a_i meters.

Chokudai is a mayor of the city, and he loves colorful thigs. And now he wants to see at least K buildings from the left.

You can increase height of buildings, but it costs 1 yens to increase 1 meters. It means you cannot make building that height is not integer.

You cannot decrease height of buildings.

Calculate the minimum cost of satisfying Chokudai's objective.

Note: "Building i can see from the left" means there are no j exists that (height of building j) ≥ (height of building i) and j < i.

Input Format

N K
a_1 a_2 a_3 ... a_N

Output Format

Print the minimum cost in one line. In the end put a line break.

Constraints

1 ≤ K ≤ N ≤ 15

1 ≤ a_i ≤ 10^9

Scoring

Subtask 1 [120 points]

N = K

Subtask 2 [90 points]

N ≤ 5

a_i ≤ 7

Subtask 3 [140 points]

There are no additional constraints.

Sample Input 1

5 5
3949 3774 3598 3469 3424

Sample Output 1

1541

The optimal solution is (height of buildings from the left) = [3949, 3950, 3951, 3952, 3953].

Sample Input 2

5 3
7 4 2 6 4

Sample Output 2

7

The optimal solution is (height of buildings from the left) = [7, 8, 2, 9, 4].

Solution in Haskell (GHC 7.10.3) (judge verdict: Wrong Answer)

import Control.Monad

solve :: Int -> [Integer] -> Integer -> Integer -> Integer
solve 0 as hd cnt = cnt
solve _ [] _ _ = 10^19 
solve k (a:as) hd cnt = left `min` right
  where
    left = solve k as hd cnt
    right = if hd < a then solve (k-1) as a cnt else solve (k-1) as (hd+1) (cnt+hd+1-a)

main = do
  [n,k] <- map read . words <$> getLine
  a <-  map read . words <$> getLine :: IO [Integer]

  let ans = solve (k-1) (tail a) (head a) 0
  print ans