      PROGRAM averageLength
      IMPLICIT NONE
      integer :: n
      real(8),allocatable :: x(:),y(:)
      integer :: i,a,b
      real(8) :: ans,counter
      
      
      read*,n
      ALLOCATE( x(n),y(n) )
      DO i = 1,n
        read*,x(i),y(i)
      END DO
      
      
      ans = 0
      counter = 0
      DO a = 1,n-1
        DO b = a+1,n
          ans = ans + sqrt( (x(a)-x(b))**2+(y(a)-y(b))**2 )
          counter = counter + 1
        END DO
      END DO
      
      ans = ans / counter * real(n-1)
      
      print*,ans
      
      
      END PROGRAM