double ratio=1, det;
    for(int i = 0; i < n; i++)
    {
        for(int j = 0; j < n; j++)
    {
            if(j>i)
        {     // ha az elem 0 , akkor sor csere
              if (abs(a[i][i]) < pow(10,-15))
          {
            for(int j = 0; j < n; ++j)
            {
              double s = a[i][j];
              a[i][j] = a[i+1][j];
              a[i+1][j] = s;
            }
          }
            ratio = a[j][i]/a[i][i];
                for(int k = 0; k < n; k++)
        {
                    a[j][k] -= ratio * a[i][k];
                }
            }
        }
    }