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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <iostream>
#include <cstdlib>
#include <string>
#include <algorithm>
using namespace std;

int comp(const void* a, const void* b){
	return *(int*)a - *(int*)b;
}

int main() {
	int n, k, x, y, ans = 0;
	cin >> n >> k >> x >> y;
	if(n <= k){
		for(int i = 0; i < n; i++){
			ans += x;
		}
	}else{
		for(int i = 0; i < k; i++){
			ans += x;
		}
		for(int i = 0; i < n-k; i++){
			ans += y;
		}
	}
	cout << ans << endl;
}