real,allocatable:: tv(:,:),tu(:,:)
real,allocatable::ex(:,:),ey(:,:),e(:,:)
integer,allocatable::rt(:,:)
open(20,file='potential.dat')
open(30,file='electr.dat')
!read *,nt
nt=100000
n1=500
n2=500
allocate (tv(n1,n2),tu(n1,n2))
allocate (ex(n1,n2),ey(n1,n2),e(n1,n2),rt(n1,n2))
tv=0.
tu=0.
rho=0.
rt=0

tv(50:450,300:320)=+1.
tv(50:450,180:200)=-1.

tu(50:450,300:320)=+1.
tu(50:450,180:200)=-1.

rt(50:450,300:320)=1
rt(50:450,180:200)=1

!******************************************
do k=1,nt
emax=-1.e+20

do i=2,n1-1
do j=2,n2-1

if(rt(i,j).eq.0)then
add=0.05*(tv(i+1,j)+tv(i-1,j)-4.*tv(i,j)+tv(i,j+1)+tv(i,j-1))
tu(i,j)=tv(i,j)+add
else
tu(i,j)=tv(i,j)
endif

if (abs(add).gt.emax) then
emax=add
endif

enddo
enddo

if (emax.lt.1.e-5.and.k.gt.100) then
print *,emax,k 
goto 200
endif

tv=tu
enddo
!*******************************************	
200 continue
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