682 B | 640 chars
import sys
from math import sqrt, pi, sin, factorial
inp = lambda : sys.stdin.readline()[:-1].strip()
mii = lambda x = " " : [*map(int,inp().split(x))]
mfi = lambda x = " " : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def solve():
while 1:
l = mii()
if sum(l) == 0:
break
R0, W0, C, R = l
for x in range(0, 100000):
if x * R >= W0 * C - R0:
print(x)
break
# W0 리터의 물에 R0 그램
# R 그램에 농도를 C로 만들려한다.
# 농도 (R0 + X * R) / (W0 + )
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
ret = solve()