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)