import bisect
import copy
import decimal
import fractions
import heapq
import itertools
import math
import random
import sys
import time
from collections import Counter,deque,defaultdict
from functools import lru_cache,reduce
from heapq import heappush,heappop,heapify,heappushpop,_heappop_max,_heapify_max
def _heappush_max(heap,item):
    heap.append(item)
    heapq._siftdown_max(heap, 0, len(heap)-1)
def _heappushpop_max(heap, item):
    if heap and item < heap[0]:
        item, heap[0] = heap[0], item
        heapq._siftup_max(heap, 0)
    return item
from math import gcd as GCD
read=sys.stdin.read
readline=sys.stdin.readline
readlines=sys.stdin.readlines

def Tersect(L,R,f,eps=1e-12,cnt=0,maximize=False):
    f_L,f_R=f(L),f(R)
    if cnt:
        for _ in range(cnt):
            l,r=(2*L+R)/3,(L+2*R)/3
            f_l,f_r=f(l),f(r)
            if maximize:
                if f_l>=f_r>=f_R:
                    R=r
                    f_R=f_r
                elif f_L<=f_l<=f_r:
                    L=l
                    f_L=f_l
                else:
                    L,R=l,r
                    f_L,f_R=f_l,f_r
            else:
                if f_l<=f_r<=f_R:
                    R=r
                    f_R=f_r
                elif f_L>=f_l>=f_r:
                    L=l
                    f_L=f_l
                else:
                    L,R=l,r
                    f_L,f_R=f_l,f_r
    else:
        while (R-L)/max(L,R,1)>=eps:
            l,r=(2*L+R)/3,(L+2*R)/3
            f_l,f_r=f(l),f(r)
            if maximize:
                if f_l>=f_r>=f_R:
                    R=r
                    f_R=f_r
                elif f_L<=f_l<=f_r:
                    L=l
                    f_L=f_l
                else:
                    L,R=l,r
                    f_L,f_R=f_l,f_r
            else:
                if f_l<=f_r<=f_R:
                    R=r
                    f_R=f_r
                elif f_L>=f_l>=f_r:
                    L=l
                    f_L=f_l
                else:
                    L,R=l,r
                    f_L,f_R=f_l,f_r
    return L

xa,ya=map(int,readline().split())
xb,yb=map(int,readline().split())
xa=decimal.Decimal(xa)
ya=decimal.Decimal(ya)
xb=decimal.Decimal(xb)
yb=decimal.Decimal(yb)
def f(y):
    return (xa**2+(ya-y)**2)**decimal.Decimal(0.5)+(xb**2+(yb-y)**2)**decimal.Decimal(0.5)
ans=Tersect(decimal.Decimal(-1<<30),decimal.Decimal(1<<30),f)
print(ans)