program interaction
      implicit none
      integer,parameter :: N = 200
      integer i,j,ij
      double precision x(N**2),y(N**2),q(N**2),dx,dy,r,e
      ij=0
      do i=1, N
        do j=1, N
          ij=ij+1
          x(ij) = dble(i)
          y(ij) = dble(j)
          q(ij) = 2.d0*(mod(ij,2)-0.5d0)
        end do
      end do
      e=0.d0
      do i=1,N**2-1
        do j=i+1,N**2
          dx=x(i)-x(j)
          dy=y(i)-y(j)
          r=dsqrt(dx*dx+dy*dy)
          e=e+q(i)*q(j)/r
        end do
      end do
      print *, e
      end program