Triangular number is the amount of points that can fill equilateral triangle. 
Example: the number 6 is a triangular number because all sides of a triangle has the same amount of points.
```
Hint!
T(n) = n * (n + 1) / 2,
n - is the size of one side.
T(n) - is the triangular number.
```
Given a number 'T' from interval [1; 2147483646], find if it is triangular number or not.
def is_triangular(n):
    return ((-1 + (1 + 8 * n)**.5) / 2) % 1 == 0

def is_triangular(n):
    return ((-1 + (1 + 8 * n)**.5) / 2) % 1 == 0
