xa, ya = map(int, input().split())
xb, yb = map(int, input().split())

xb *= -1
p = xb / (xb - xa)
print(p * ya + (1-p) * yb)

# x, y = p(xa, ya) + (1-p) * (xb, yb)
# x = p*xa + (1-p)*xb = 0
# (xb-xa)*p = xb
# p = xb/(xb-xa)
# y = p*ya + (1-p)*yb