Triangular Number Sequence

This Triangular Number Sequence is generated from a pattern of dots that form a triangle. The first 5 numbers of the sequence, or dots, are:

1, 3, 6, 10, 15

This means that the first triangle has just one dot, the second one has three dots, the third one has 6 dots and so on.

Write a function that returns the number of dots when given its corresponding triangle number of the sequence.

triangle(1) ➞ 1
triangle(6) ➞ 21
triangle(215) ➞ 23220