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: Wrong Answer)

#include <stdio.h>
#include <string.h>

int main(void){
	char s[100];
	int i,j,len;
	int tagi[26] = {0};
	char tagc[26] = "abcdefghijklmnopqrstuvwxyz";
	scanf("%s",s);
	len = strlen(s);
	for(i=0;i<len;i++){
		for(j=0;j<26;j++){
			if(tagc[j] == s[i]){
				tagi[j]++;
				break;
			}
		}
	}
	for(i=0;i<26;i++){
		if(tagi[i]%2!=0){
			printf("No\n");
			return -1;
		}
	}
	printf("Yes\n");
	return 0;
}