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) {
  var args = input.split('\n');
  var n = parseInt(args[0], 10);
  var k = parseInt(args[1], 10);
  var x = parseInt(args[2], 10);
  var y = parseInt(args[3], 10);
  
  if (n <= k) {
    var ans = n * x;
  } else {
    var ans = k * x + (n - k) * y;
  }

  console.log(ans);
}

main(require('fs').readFileSync('/dev/stdin', 'utf8'));