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>
#include<stdlib.h>
#include<string.h>
//絶対値
long long int change_plus(long long int a) {
	if (a < 0) { return -a; }
	if (a >= 0) { return a; }
}
//大小比較(大)
long long int big(long long int a, long long int b) {
	if (a >= b) {
		return a;
	}
	else {
		return b;
	}
}

//大小比較(小)
long long int small(long long int a, long long int b) {
	if (a >= b) {
		return b;
	}
	else {
		return a;
	}
}

int main() {
	long long int n, m, k,p,q;
	char str[30],str2[15],str3[15];
	scanf("%lld%lld%lld%lld",&n,&m,&k,&p);
	if (n <= m) { printf("%lld", n*k); }else{printf("%lld",(n-m)*p+k*m); }
	
	
}