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<vector>
#include<algorithm>
#include<iomanip>

#define PI 3.141592653589793238
#define INF 1050000000

using namespace std;



//cout << fixed << setprecision(15) << << endl;
//cout << << endl;

int main() {

	int N,K,X,Y;

	cin >> N >> K >> X >> Y;

	int ans = 0;

	if (N <= K) {
		for (int i = 0; i < N; i++) {
			ans += X;
		}

		cout << ans << endl;
	}
	else {
		for (int i = 0; i < K; i++) {
			ans += X;
		}
		for (int i = K; i < N; i++) {
			ans += Y;
		}

		cout << ans << endl;
	}

	return 0;
}