# -*- coding: utf-8 -*-
"""
No.306 さいたま2008
https://yukicoder.me/problems/no/306

"""
import sys
from sys import stdin
input = stdin.readline


def calc_dist(ax, ay, bx, by, py):
    ap = (ax**2 + (ay - py)**2)**0.5
    bp = (bx**2 + (by - py)**2)**0.5
    return ap + bp


def solve(ax, ay, bx, by):
    uy = max(ay, by)
    ly = min(ay, by)

    if uy == ly:
        return ly

    for _ in range(100):
        py1 = (ly*2 + uy) / 3
        mid1_score = calc_dist(ax, ay, bx, by, py1)
        py2 = (ly + uy*2) / 3
        mid2_score = calc_dist(ax, ay, bx, by, py2)
        if mid2_score > mid1_score:
            uy = py2
        else:
            ly = py1
    return (uy + ly) / 2


def main(args):
    ax, ay = map(float, input().split())
    bx, by = map(float, input().split())
    ans = solve(ax, ay, bx, by)
    print('{:8f}'.format(ans))


if __name__ == '__main__':
    main(sys.argv[1:])