xa, ya = map(int, input().split())
xb, yb = map(int, input().split())
yp = 0
if ya > yb:
    yp = yb + xa * (ya - yb) / (xa + xb)
    print(yp)
else:
    yp = ya + xa * (yb - ya) / (xa + xb)
    print(yp)