vl, vr = map(int, input().split())
d = int(input())
w = int(input())

total_time = d / (vl + vr)
total_distance = w * total_time

# Formatting the output to have up to 10 decimal places and removing trailing zeros
formatted_output = "{0:.10f}".format(total_distance).rstrip('0').rstrip('.') if '.' in "{0:.10f}".format(total_distance) else "{0:.0f}".format(total_distance)
print(formatted_output)