real,allocatable:: tv(:,:),tu(:,:),rho(:,:)
real,allocatable::ex(:,:),ey(:,:),e(:,:)
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))
tv=0.
tu=0.
rho=0.

do i=1,n1
do j=1,n2
ii=i-250
jj=j-250
if((ii*ii+jj*jj).le.2500) then
rho(i,j)=1.
endif
enddo
enddo

!******************************************
do k=1,nt

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

tv=tu

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
100format (500 e15.6)
write (20,100) tu
write (30,100) e

stop
end