
Score : 100 points
Problem StatementThere 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.
InputThe input is given from Standard Input in the following format:
N
K
X
Y
OutputPrint Tak's total accommodation fee.
Sample Input 15
3
10000
9000
Sample Output 148000
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 22
3
10000
9000
Sample Output 220000
