import math
import random
import sys
import numpy as np

if (len(sys.argv)<3):
 print 'too few input parameters, format:'
 print 'python distparabola.py p x1 y1 '
 print 'example:'
 print 'python distparabola.py 1 1 0'
 #sys.exit()
 p = 1
 x1 = 1
 y1 = 0
 print 'default values used.'
 print ' '
else:
 p = float(sys.argv[1])
 x1 = float(sys.argv[2])
 y1 = float(sys.argv[3])

TwoPi=math.pi*2

def sign(x):
   if abs(x)<1e-6:
       return 0
   elif x>0:
       return 1
   else:
       return -1

# Function from https://www.douban.com/note/523859088/
def getCubicRoot(a,b,c,d):
   a=float(a)
   b=b/(3.0*a)
   c=c/(6.0*a)
   d=d/(2.0*a)
   
   solve=[None,None,None]
   Alph=-b**3+3.0*b*c-d
   Beta=b**2-2.0*c
   Delt=Alph**2-Beta**3

   if (Delt)>0.0:
       tht=Alph+math.sqrt(Delt)
       R1=abs(abs(tht)**(1.0/3.0))*sign(tht)
       tht=Alph-math.sqrt(Delt)
       R2=abs(abs(tht)**(1.0/3.0))*sign(tht)
       solve[0]=-b+R1+R2
   elif abs(Delt)<1e-6:
       R1=abs(abs(Alph)**(1.0/3.0))*sign(Alph)
       if abs(R1)<1e-6:
           solve[0]=-b
       else:
           solve[0]=-b+2.0*R1
           solve[1]=-b-R1
   elif Delt<0.0:
       tht=math.acos(Alph/(math.sqrt(Beta)*Beta))
       solve[0]=-b+2.0*math.sqrt(Beta)*math.cos(tht/3.0)
       solve[1]=-b+2.0*math.sqrt(Beta)*math.cos((tht+TwoPi)/3.0)
       solve[2]=-b+2.0*math.sqrt(Beta)*math.cos((tht-TwoPi)/3.0)
   return solve

c=-2.0*p*(y1-p)
d=-2.0*p**2*x1
a=1
b=0


ans=getCubicRoot(a,b,c,d)
#print ans
x0=ans[0]
y0=2.0*p*x0**2
dist=np.sqrt((x1-x0)**2+(y1-y0)**2)
print dist