# -*- coding: utf-8 -*-
import sys,copy,math,heapq,itertools as it,fractions,re,bisect,collections as coll

def dist(y):
    res = math.sqrt((ya - y)**2 + xa**2) + math.sqrt((yb- y)**2 + xb**2)
    return res

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

low, hi = min(ya, yb), max(ya, yb) 
while hi - low > 1e-11:
    mid_l = (2*low + hi)/3.0
    mid_r = (low + 2*hi)/3.0
    d_l = dist(mid_l)
    d_r = dist(mid_r)
    if d_l < d_r:
        hi = mid_r
    else:
        low = mid_l
print "%.10f" % ((low + hi)/2.0)