<image> It is known that as shown in the figure, there is a triangular dot array with infinitely many rows. The first row has one dot, the second row has two dots, ..., the n-th row has n dots. It is easy to find that the sum of the dots in the first 4 rows of the triangular dot array is 10. If the sum of the dots in the first a rows of the triangular dot array is 300, then the value of a is ______.