r0,w,c,r = gets.split.map(&:to_i)

ans = []
while r0 != 0 && w != 0 && c != 0 && r != 0 
	i = 0
	while (r0/w) != c 
		if (r0/w) > c 
			puts "0"
			break
		end
		i += 1
		#tmp_y = (r0+i*r) / c
		tmp = r0/w
		#tmp_y = r0+i*r
		if c <= (r0+i*r)/w
			#ans.push(i)
			puts i
			break
		end
	end
	puts "0" if r0/w == c
	r,w,c,r = gets.split.map(&:to_i)
end
=begin
for i in ans
	puts i 
end
=end