Problem: SmallR is an archer. SmallR is taking a match of archer with Zanoes. They try to shoot in the target in turns, and SmallR shoots first. The probability of shooting the target each time is <image> for SmallR while <image> for Zanoes. The one who shoots in the target first should be the winner.

Output the probability that SmallR will win the match.

Input

A single line contains four integers <image>.

Output

Print a single real number, the probability that SmallR will win the match.

The answer will be considered correct if the absolute or relative error doesn't exceed 10 - 6.

Examples

Input

1 2 1 2


Output

0.666666666667

Solution (in PYTHON3):

Solution:
from math import *
#from bisect import *
#from collections import *
#from random import *
#from decimal import *"""
#from heapq import *
#from random import *
import sys
input=sys.stdin.readline
#sys.setrecursionlimit(3*(10**5))
global flag
def inp():
    return int(input())
def st():
    return input().rstrip('\n')
def lis():
    return list(map(int,input().split()))
def ma():
    return map(int,input().split())
t=1
while(t):
    t-=1
    a,b,c,d=ma()
    r=a/b
    de=1-((1-(a/b))*(1-(c/d)))
    print(r/de)