sum = 0
money = 0
v = int(input("请输入最开始的价格v："))
m = int(input("请输入每录取m人之后提升价格"))
a = int(input("请输入价格输入a："))
n = int(input("最终收多少人"))
for i in range(n //m):
money =int( m * v)
sum = sum + money
v = v + a
sum = int(sum + (n // m + 1) * (a * n % m))
print(sum)
