program main

 implicit none

 integer              :: n,mm,i,j
 real*8,  allocatable :: a(:,:),b(:,:),c(:,:)
 real*8               :: na,nb
 logical              :: lnorm

 !1)
 do 
   write(*,*) 'Dimension n>1'
   read(*,*) n
   if (n.gt.1) exit
 enddo

 allocate(a(n,n),b(n,n),c(n,n))

 do i=1,n
    do j=1,n
       read(*,*) a(i,j),b(i,j)
    enddo
 enddo   

 !2)
 call norm(a,b,n,na,nb)
 if (na.eq.nb) then
    write(*,*) "A and B get the same norm"
    lnorm=.true.
 else   
    write(*,*) "A and B do not get the same norm"
    lnorm=.false. 
 endif        

 !3)
 call define_c(lnorm,a,b,n,na,nb,c)

 !4)

 open(10,file='norm.dat')
 write(10,'("Norm of A is", F10.3,", norm of B is", F10.3)') na, nb
 close(10)

 open(10,file='c.dat')
 do i=1,n
    do j=1,n
       write(10,95) i,j,c(i,j)
    enddo
 enddo    
 close(10)

 deallocate(a,b,c)

95  format(I5,I5,F10.3)

 stop

end program main      

subroutine norm(a,b,n,na,nb)
 
 implicit none

 integer, intent(in)  :: n
 real*8,  intent(in)  :: a(n,n),b(n,n)
 real*8,  intent(out) :: na, nb

 integer              :: i,j

 na=0.d0
 nb=0.d0
 do i=1,n
    do j=1,n
       na=na+abs(a(i,j))**2
       nb=nb+abs(b(i,j))**2 
    enddo
 enddo   
 na=dsqrt(na)
 nb=dsqrt(nb)

 return 

end subroutine norm        

subroutine define_c(lnorm,a,b,n,na,nb,c)

 implicit none

 integer, intent(in)  :: n
 real*8,  intent(in)  :: a(n,n),b(n,n)
 real*8,  intent(in)  :: na, nb
 logical, intent(in)  :: lnorm
 real*8,  intent(out) :: c(n,n)

 integer              :: i,j
 real*8               :: diff_norm 

 c=0.d0
 if (lnorm) then
    do i=1,n
       do j=1,n
          c(i,j) = a(i,j) + b(i,j) + na
       enddo
    enddo   
 else
    diff_norm=abs(na-nb)     
    c=0.d0 
    do i=1,n
       c(i,i) = diff_norm 
    enddo   
    do i=2,n
       c(i,i-1) =  2.d0*diff_norm
    enddo    
    do i=1,n-1
       c(i,i+1) = 0.5d0*diff_norm 
    enddo   
 endif

 return 

end subroutine define_c