## A Baker's Dozen

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Points: 3
Time limit: 2.0s
Memory limit: 64M

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Problem type

When ordering a quantity of pastries, a baker's dozen is considered to be 13. At TSS Bakery, you are charged the cost of a dozen pastries when you purchase a baker's dozen, effectively giving you 1 for free. In addition to a separate cost for a dozen, there is another cost for individual pastries below a dozen. Given the costs as well as the number of pastries purchased, determine the final amount of the bill.

#### Input Specification

The first line of input is an integer representing the cost of a baker's dozen pastries.
The second line of input is an integer representing the cost of 12 (or a dozen) pastries.
The third line of input is an integer representing the cost of a single pastry.
The last line is an integer representing the number of pastries purchased.

When calculating the total amount, the cost for a baker's dozen is charged first until the amount of pastries unpaid is less than 13. If the amount remaining is 12, the cost for a dozen is charged. If the amount remaining is less than 12, the remaining cost is the individual cost multiplied by the number of pastries remaining.

#### Output Specification

Output a single integer, the cost of pastries.

#### Sample Input

10
9
3
41

#### Sample Output

36

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