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)

# 1 軒のホテルがあります。 このホテルの宿泊費は、次のようになっています。
# 最初の K 泊までは、 1 泊あたり X 円 K + 1 泊目以降は、 1 泊あたり Y 円 高橋君は、このホテルに N 泊連続で宿泊することにしました。
# 高橋君の宿泊費は合計で何円になるか求めてください。

N = int(input())
K = int(input())
X = int(input())
Y = int(input())

if N - K >= 1:
    print((X * K) + (Y * (N - K)))

else:
    print(X * N)