import sys

read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline

def from_read():
    return [int(x) for x in read().splitlines()]

def from_readline():
    return [int(x) for x in readline().split()]

def read_matrix(N, M):
    matrix = [None for i in range(N)]
    for i in range(N):
        matrix[i] = from_readline()
    return matrix

def solve(xa, ya, xb, yb):
    l = -100
    r = 1000
    for _ in range(300):
        c1 = (2*l + r) / 3
        c2 = (l + 2*r) / 3
        L1 = (xa**2 + (c1 - ya)**2)**.5 + (xb**2 + (c1 - yb)**2)**.5
        L2 = (xa**2 + (c2 - ya)**2)**.5 + (xb**2 + (c2 - yb)**2)**.5
        if L1 > L2: l = c1
        else: r = c2
    return l

def main():
    xa, ya = from_readline()
    xb, yb = from_readline()
    print(solve(xa, ya, xb, yb))

if __name__ == "__main__":
    main()