Problem: John is travelling by train, The fare of the train depends on the kilometer he traveled. For the first A kilometer, the fare is M and for the subsequent distance, it is N per kilometer. You are given total distance covered by train. Find out the total fare.

Input: Each input contains four space separated Integer A,M,N,D(total distance).

Output: Print the value of total fare.

Constraints: 1<=A,M,N,D<=1000

Sample Input: 1 2 3 5

Sample Output: 

14

a, m, n, d = map(int, input().split())
print(min(a, d) * m + max(d - a, 0) * n)