# Pastebin uDpP9J6s
'''
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John is travelling by train.
The fare depends on the kilometers traveled.
For the first A kilometers, the fare is M.
For the subsequent distance, it's N per kilometer.
The total distance covered by train is D.
Find out the total fare.
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INPUT
Four space separated integers A,M,N,D.
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OUTPUT
Print the value of total fare.
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CONSTRAINTS
1<=A,M,N,D<=1000
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SAMPLE INPUT
1 2 3 5
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SAMPLE OUTPUT
14
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'''

A,M,N,D = [int(x) for x in input().split(' ')]
print(A*M+N*(D-A))