# coding: utf-8
##yuki_236

s=[]
s=map(int,raw_input().split())
lst=list(s)
ls=lst[:4]

A=ls[0]*1.0
B=ls[1]*1.0
X=ls[2]*1.0
Y=ls[3]*1.0

if X/Y>A/B:
 res=A/B*Y+Y
elif X/Y<A/B:
 res=X+B/A*X
else:
 res=X+Y

print res