real,allocatable:: tv(:,:),tu(:,:),rho(:,:)
        real,allocatable::ex(:,:),ey(:,:),e(:,:)
        integer,allocatable::rt(:,:)
	open(20,file='potential.dat')
	open(30,file='electr.dat')
	read *,nt
	n1=500
	n2=500
	allocate (tv(n1,n2),tu(n1,n2),rho(n1,n2))
        allocate (ex(n1,n2),ey(n1,n2),e(n1,n2),rt(n1,n2))
	do i=1,n1
	do j=1,n2
	tv(i,j)=0.
	tu(i,j)=0.
	rho(i,j)=0.
	rt(i,j)=0
	enddo
	enddo
	
	do i=50,450
	
	do j=300,320
	tv(i,j)=+1.
	rt(i,j)=1
	enddo
	
	do j=180,200
	tv(i,j)=-1.
	rt(i,j)=1
	enddo
	
	enddo
	
!******************************************
	do k=1,nt
	
	do i=2,n1-1
	do j=2,n2-1
	if(rt(i,j).eq.0)then
	tu(i,j)=tv(i,j)+rho(i,j)+
     *	0.05*((tv(i+1,j)+tv(i-1,j)-4.*tv(i,j))+tv(i,j+1)+tv(i,j-1))
        else
        tu(i,j)=tv(i,j)
        endif
	enddo
	enddo

	do i=1,n1
	do j=1,n2
	tv(i,j)=tu(i,j)
	enddo
	enddo

	
	enddo
!*******************************************	
	do i=2,n2-1
	do j=2,n1-1
	ex(i,j)=tu(i+1,j)-tu(i-1,j)
	ey(i,j)=tu(i,j+1)-tu(i,j-1)
	e(i,j)=sqrt(ex(i,j)**2+ey(i,j)**2)
	enddo
	enddo
100	format (500 e15.6)
	write (20,100) tu
	write (30,100) e

	stop
	end