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:
######################################################
############Created by Devesh Kumar###################
#############devesh1102@gmail.com####################
##########For CodeForces(Devesh1102)#################
#####################2020#############################
######################################################
import sys
input = sys.stdin.readline
 
# import sys
import heapq 
import copy
import math
import decimal
 
# import sys.stdout.flush as flush
# from decimal import *
#heapq.heapify(li) 
#
#heapq.heappush(li,4) 
#
#heapq.heappop(li)
#
# &    Bitwise AND Operator    10 & 7 = 2
# |    Bitwise OR Operator    10 | 7 = 15
# ^    Bitwise XOR Operator    10 ^ 7 = 13
 
# <<    Bitwise Left Shift operator    10<<2 = 40
# >>    Bitwise Right Shift Operator
# '''############ ---- Input Functions ---- #######Start#####'''
 
 
def inp():
    return(int(input()))
def inlt(): 
    return(list(map(int,input().split())))
def insr():
    s = input()
    return(list(s[:len(s) - 1]))
def insr2():
    s = input()
    return((s[:len(s) - 1]))
def invr():
    return(map(int,input().split()))
############ ---- Input Functions ---- #######End
# #####   
 

ans = 0
def pr_list(a):
    print( *a , sep=" ")
def main():
    # tests =  inp()
    tests = 1
    mod = 1000000007
    limit = 10**18
    ans = 0
    stack = []
    hashm = {}
    arr = []
    heapq.heapify(arr) 

    for test in range(tests):
        # l1 = insr()(
        [a,b,c,d] = inlt()
        print( abs((a/b)*(1/ (1 - (1 - (a/b)) * (1 - (c/d)) ))) )
        
    




if __name__== "__main__":
    main()