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 JavaScript (node.js v5.12) (judge verdict: Accepted)

function main(input) {
  input=input.trim();
  const n = parseInt(input.split("\n")[0], 10);
  const k = parseInt(input.split("\n")[1], 10);
  const x = parseInt(input.split("\n")[2], 10);
  const y = parseInt(input.split("\n")[3], 10);

  if(n>k){
    console.log(k*x+(n-k)*y);
  }else{
    console.log(n*x);
  }
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));