A, B, C, D, E = map(int, input().strip().split())

Ame_num = A*(B-C)
X=D
Ame_cost=0

for i in range(Ame_num):

    if i%10 != 0:
        Ame_cost = Ame_cost + X

    elif i%10 == 0 and E <= X:
        X = X - E
        Ame_cost = Ame_cost + X
    
    elif i%10 == 0 and E > X:
        Ame_cost = Ame_cost + X

    else: pass

print(Ame_cost)