program kadai
    implicit none
    integer  ::  N
real(8),allocatable  ::  x(:),y(:)
integer  ::  i,kake,j
real(8)  ::  kyori
real(8)  ::  ans

read(*,*) N

allocate(x(N))
allocate(y(N))

do i=1,N
    read(*,*) x(i),y(i)
end do

kyori=0
do i=1,N-1
    do j=i+1,N
        kyori = kyori + (sqrt((x(i)-x(j))**2+(y(i)-y(j))**2))
    end do
end do

ans = kyori*2/(N)
write(*,*) ans

stop
contains
end program kadai