A Bullet is fired vertically upwards with velocity $v$ from the surface of a spherical planet. When it reaches its maximum height, its acceleration due to the planet's gravity is $1 / 4^{\text {th }}$ of its value at the surface of the planet. If the escape velocity from the planet is $v_{\text {esc }}=v \sqrt{n}$, then the value of $N$ is (ignore energy loss due to atmosphere)