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)

strToInt :: String -> Int
strToInt x = read x :: Int

leadAns :: Int -> Int -> Int -> Int -> Int
leadAns n k x y 
    | n==1 = x
    | n > k = y + leadAns (n-1) k x y
    | otherwise = x + leadAns (n-1) k x y

main = do
    inn <- getLine
    ink <- getLine
    inx <- getLine
    iny <- getLine
    let n = read inn :: Int
        k = read ink :: Int
        x = read inx :: Int
        y = read iny :: Int
    putStrLn $ show $ leadAns n k x y