'''
Bob has recently started commuting by bus. We know that a one bus ride ticket costs a dollars. Besides, Bob found out that he can buy a special ticket for m rides, 
also he can buy it several times. It costs b dollars. Bob will need to use bus n times. Help Bob to calculate what is the minimum sum of money he will have to spend 
to make n rides?

Example 1
Input:
n = 10; m = 3; a = 1; b = 2
Output:
7
Example 2
Input:
n = 10; m = 2; a = 1; b = 1
Output:
5
'''
import math

class Solution(object):
    def solve(self, n: int, m: int, a: int, b: int) -> int:  # n: number of rides, m: rides in special ticket, a: cost of single ticket, b: cost of special ticket
        # Calculate the cost using only single tickets
        cost_single = n * a
        # Calculate the cost using special tickets
        cost_special = (n // m) * b + (n % m) * a   # Full special tickets + remaining rides with single tickets
        
        cost_packs_only = math.ceil(n / m) * b  # If we only use special tickets, we need to round up the number of packs
        # Return the minimum of the two costs
        return min(cost_single, cost_special, cost_packs_only)
    

s = Solution()
print(s.solve(10, 3, 5, 1))  # Output: 4
print(s.solve(10, 2, 1, 1))  # Output: 5