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

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


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


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

    dax = Decimal(ax)
    day = Decimal(ay)
    dbx = Decimal(bx)
    dby = Decimal(by)
    getcontext().prec = 28

    for _ in range(100):
        py1 = Decimal((ly*2 + uy) / 3)
        mid1_score = calc_dist(dax, day, dbx, dby, py1)
        py2 = Decimal((ly + uy*2) / 3)
        mid2_score = calc_dist(dax, day, dbx, dby, 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('{:f}'.format(ans))


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