Staying in the triangle

Probability Level 4

Three points are chosen uniformly at random from the boundary of a square and a fourth point is chosen uniformly at random from the interior. The probability that the 4th point lies in the triangle formed by the other 3 points can be expressed as $\frac{a}{b}$ where $a$ and $b$ are coprime positive integers. What is the value of $a + b?$