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 Python (3.4.3) (judge verdict: Wrong Answer)

# -*- coding: utf-8 -*-
# !/usr/bin/env python
# vim: set fileencoding=utf-8 :

"""
#
# Author:   Noname
# URL:      https://github.com/pettan0818
# License:  MIT License
# Created: 火 11/ 8 16:49:06 2016

# Usage
#
"""
def read_input():
    N = int(input())
    K = int(input())
    X = int(input())
    Y = int(input())

    return N, K, X, Y


def check_sum(N, K, X, Y):
    """
    >>> check_sum(5, 3, 10000, 9000)
    48000
    >>> check_sum(2, 3, 10000, 9000)
    20000
    """
    if N > K:
        return K * X + (N-K)*Y
    else:
        return N * X


if __name__ == "__main__":
    import doctest

    doctest.testmod()
    N, K, X, Y = read_input()

    check_sum(N, K, X, Y)