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:
import sys
def input():
    return sys.stdin.readline().strip()
 
 
def iinput():
    return int(input())
 
 
def tinput():
    return input().split()
 
 
def rinput():
    return map(int, tinput())
 
 
def rlinput():
    return list(rinput())

a,b,c,d = rinput()
print((a/b)/(1-((1-a/b)*(1-c/d))))