for (i=0 ;i<N; i++)  
    for (j=0 ;j<N; j++) {
      c[i][j] = 0;
      a[i][j] = drand48();
      b[i][j] = drand48();
    }



  for (i=0; i<N; i++) 
    for (j=0; j<N; j++) 
      for (k=0; k<N; k++) c[i][j] = c[i][j] + a[i][k]*b[k][j];


  for (i=1; i<=NN+1; i++) {
    x[i][0] = a5*i;
    x[i][0] = a5*i;

    x[i][NN+1] = a6*i;
    x[i][NN+1] = a6*i;

    x[NN+1][i] = 0.0;
    x[NN+1][i] = 0.0; 

    y[NN+1][i] = 0.0;
    y[NN+1][i] = 0.0; 
  }

    maxdiff1 = -1.0;
    for (i=1; i <= NN; i++) 
      for (j=1; j <=NN; j++) {
        y[i][j] = a1*x[i-1][j] + a3 * x[i+1][j] +
                     a2*x[i][j-1] + a4 * x[i][j+1];
        if (myabs(y[i][j] - x[i][j]) > maxdiff1)
          maxdiff1 = myabs(y[i][j] - x[i][j]);
      }

  for (i=0; i<1000; i++) {
    c[i] = c[i-1]+1.0;


    for (i=1; i <= NN; i++) 
      for (j=1; j <=NN; j++) {
        x[i][j] = a1*x[i-1][j] + a3 * x[i+1][j] +
                     a2*x[i][j-1] + a4 * x[i][j+1];