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 C (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
 
int main(void)
{
  int N,K,X,Y;
  scanf("%d\n%d\n%d\n%d",&N,&K,&X,&Y);
  if (1 <= N && N <= 10000 && 1 <= K && K <= 10000 && 1 <= Y && Y < X && X <= 10000) {
    if (N > K) {
      printf("%d\n",K * X + (N - K) * Y);
    } else {
      printf("%d\n",N * X);
    }
  }
  return 0;
}