from decimal import Decimal
import math
x1,y1=list(map(Decimal,input().split()))
x2,y2=list(map(Decimal,input().split()))
de=Decimal(math.sqrt((x1-x2)**2+(y1-y2)**2))
dm=abs(x1-x2)+abs(y1-y2)
x=abs(de-dm)
print(x)