Problem:
Score : 100 points

Problem Statement

There is a hotel with the following accommodation fee:

X yen (the currency of Japan) per night, for the first K nights

Y yen per night, for the (K+1)-th and subsequent nights

Tak is staying at this hotel for N consecutive nights.
Find his total accommodation fee.

Constraints

1 \leq N, K \leq 10000

1 \leq Y < X \leq 10000

N,\,K,\,X,\,Y are integers.

Input

The input is given from Standard Input in the following format:

N
K
X
Y

Output

Print Tak's total accommodation fee.

Sample Input 1

5
3
10000
9000

Sample Output 1

48000

The accommodation fee is as follows:

10000 yen for the 1-st night

10000 yen for the 2-nd night

10000 yen for the 3-rd night

9000 yen for the 4-th night

9000 yen for the 5-th night

Thus, the total is 48000 yen.

Sample Input 2

2
3
10000
9000

Sample Output 2

20000

Solution in Haskell (GHC 7.10.3) (judge verdict: Accepted)

import Data.List
import Control.Monad

day :: [String]
day = ["SUN", "MON", "TUE", "WED", "WED", "THU", "FRI", "SAT"]

main = print =<< ((pure calc) <*> getInts)

getInt :: IO Int 
getInt = (pure read) <*> getLine

getInts :: IO [Int]
getInts = replicateM 4 getInt

getx :: [Int] -> Int
getx [n, k, x, _] = (*) x $ min n k

gety :: [Int] -> Int
gety [n, k, _, y] = (*) y $ max 0 $ n - k

calc :: [Int] -> Int
calc val = (getx val) + (gety val)