x1,y1=map(int,input().split(' '))
x2,y2=map(int,input().split(' '))
d1=((x1-x2)**2+(y1-y2)**2)**0.5
d2=((x1-x2)**2)**0.5+((y1-y2)**2)**0.5
z = ((d1-d2)**2)**0.5
print(z)

绝对值就是平方后再求平方根