Vitya is connected to the Internet according to the following tariff plan. The monthly subscription fee is A rubles, and this subscription fee includes B megabytes of traffic. Unused megabytes "burn out" at the end of the month. If the traffic exceeds B megabytes, then each megabyte of traffic in excess of prepaid costs C rubles.
It is known that last month Vitya spent D megabytes of traffic. How much did Internet access cost him last month (including the monthly fee)?
Input
Four natural numbers A, B, C, D are entered. All numbers do not exceed 100.
Imprint
Print one number — the amount (in rubles) that Vitya has to pay for the Internet.
Examples
| # |
Input |
Output |
| 1 |
100 10 12 15 |
160 |
| 2 |
100 10 12 1 |
100 |