Gauss Seidal Method

program g_seidal
    implicit none
    integer, parameter::n=3, M=100
    integer::i,j,k
    real::a(n,n+1),b(n),x(n),x0(n), tol=0.00001,s1,s2,norm
    open(11, file="in.txt")
    ! reading matrix
    do i=1,n
        read(11,*) (a(i,j), j=1,n+1)
    end do
    ! reading initial guess
    read(11,*) (x0(i), i=1,n)
    ! separating b
    do i=1,n
        b(i)= a(i,n+1)
    end do

    do k=1,M
        do i=1,n
            s1=0.0
            do j=1,i-1
                s1=s1+a(i,j)*x(j)
            end do
            s2=0.0
            do j=i+1,n
                s2=s2+a(i,j)*x0(j)
            end do
            x(i) = ((-s1-s2+b(i))) / a(i,i)
        end do
        !write(*,*) (x(i), i=1,n)
        if(norm(x,x0)<tol) then
            write(*,*) (x(i), i=1,n)
            STOP
        end if
        do i=1,n
            x0(i)=x(i)
        end do

    end do
    write(*,*) (x(i), i=1,n)

end program g_seidal

function norm(x,x0)
    integer,parameter::n=3
    integer::i
    real::x(n),x0(n),s,norm
    s=0.0
    do i=1,n
        s=s+(x(i)-x0(i))**2
    end do
    norm = SQRT(s)
end function