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: Runtime Error)

/*  ex_final-2d
 motonaga.y.aa
 Yuto Motonaga */

#include <stdio.h>
#define HMAX 19
#define WMAX 19

int main(void) {
    char gridchar[HMAX][WMAX];                  //マス目の文字、記号(s . * tのいずれか)
    int grid[HMAX][WMAX]={0};                   //行き方の総数
    char newline;                               //改行を読み込む
    char newl_spa[HMAX][WMAX];                  //改行とスペースを読み込む
    int H, W, i, j, k, l, tmp;
    
    //データの読み込み
    scanf("%d %d", &H, &W);
    scanf("%c", &newline);
    for(i=1; i<=H; i++) {
        for(j=1; j<=W; j++) {
            scanf("%c", &gridchar[i][j]);
            scanf("%c", &newl_spa[i][j]);
        }
    }
    
    grid[1][1] = 1;
    for(i=1; i<=H; i++) {
        for(j=1; j<=W; j++) {
            grid[i][j] += grid[i-1][j] + grid[i][j-1];
            if(gridchar[i][j]=='*')
                grid[i][j] = 0;
            
            //中継マスへの行き方の総数のみを残す。
            if(gridchar[i][j]=='u') {
                tmp = grid[i][j];
                for(k=1; k<=H; k++) {
                    for(l=1; l<=W; l++) {
                        grid[k][l] = 0;
                    }
                }
                grid[i][j] = tmp;
            }
        }
    }
    
    printf("%d\n", grid[H][W]);
    return 0;
}