a, b, c, d, e = map(int, input().split())
n = a * (b - c)
x = d
ans = 0
for i in range(1, n + 1):
    if i % 1 != 0:
        ans += x
    elif e <= x:
        ans += x - e
        bef = x - e
    else:
        ans += x
print(ans)