program main
  implicit none
  
  integer :: n,xx(8)=0,yy(8)=0,i,j
  double precision :: x1,y1,r,s=0.d0,b=1.d0
  
  read(*,*)n
  do i = 1, n
    b = b * dble(i)
    read(*,*)xx(i),yy(i)
  end do
  
  do i = 1, n-1
    x1 = dble(xx(i))
    y1 = dble(yy(i))
    do j = i+1, n
      x2 = dble(xx(j))
      y2 = dble(yy(j))
      s = s + sqrt((x1 - x2)**2 + (y1 - y2)**2)
    end do
  end do
  write(*,*)s/b
end program main
  