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

if ya == yb:
    yp = ya
elif xa == xb:
    yp = (ya + yb)/2
else:
    xb = -xb
    m = (yb-ya)/(xb-xa)
    yp = m * (-xb) + yb

print(yp)