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 PYTHON):

Solution:
# Codeforces Round 185: Division 2
# Problem B: Archer

def probability(a, b, c, d):
    return float(a*d)/(a*d+b*c-a*c)

x = raw_input("").split(' ')
a = int(x[0])
b = int(x[1])
c = int(x[2])
d = int(x[3])
print probability(a, b, c, d)