def main():
    P = tuple(map(int, input().split()))
    Q = tuple(map(int, input().split()))

    if P[0] * Q[0] < 0:
        x = (abs(P[0]) + abs(Q[0])) / 2
    else:
        x = abs(P[0] - Q[0]) / 2

    if P[1] * Q[1] < 0:
        y = (abs(P[1]) + abs(Q[1])) / 2
    else:
        y = abs(P[1] - Q[1]) / 2

    print(x + y)

main()