//red halfiteration
        #pragma omp parallel
        {
            #pragma omp for private(j)
            for (int i = 1; i < n-1; i++) {
                for (j = 1; j < n-1; j++) {
                    if ( (i+j) %2 == 0) {   //optimize!
                        inout_u(i,j) = (inout_u(i-1,j) + inout_u(i+1,j)+ inout_u(i,j-1) + inout_u(i,j+1) - spacing*spacing*input_f(i,j))/4.0;
                    }
                }
            }
        }