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 (3.8.2) (judge verdict: Accepted)

# abc044_a
# https://atcoder.jp/contests/abc044/tasks/abc044_a

# 最初のK泊までは、X円
# K＋1泊目以降は、Y円

# N泊連続で宿泊した際の宿泊費

# 入力
n = int(input())
k = int(input())
x = int(input())
y = int(input())

# 処理

# x円とy円
if n > k:
    answer = k * x + (n - k) * y
# x円のみ
else:
    answer = n * x

print(answer)