题解 | #平均年龄#

import math
import traceback
while True:
    try:
        W, Y, x, N = input().strip().split()
        W, Y, N = list(map(int, [W, Y, N]))
        x = float(x)
        for i in range(1, N+1):
            Y += 1
            Y = (W*Y*(1-x)+W*x*21)/W
        print(math.ceil(Y))
    except:
        break