def ternary_search_min(l, r, evaluate):
    EPS = 10 ** (-7)
    while r - l > EPS:
        midl = l + (r - l) / 3
        midr = r - (r - l) / 3
        if evaluate(midl) > evaluate(midr):
            l = midl
        else:
            r = midr
    return l, evaluate(l)


def solve(y):
    dist = ((ya - y) ** 2 + xa ** 2) ** 0.5 + ((yb - y) ** 2 + xb ** 2) ** 0.5
    return dist


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


y, dist = ternary_search_min(-10, 1000 + 10, solve)
print(y)