The gun o'er the hill

A gun is located at the top of a hill of height $h$ above the horizontal plane.It fires two shells with the same velocity $u$ at angles $\alpha$ and $\beta$ with the horizontal.If the shells strike the horizontal ground at the same place then the value of $h$ can be expressed as $h=\frac{a u^{2} (1-\tan\alpha \tan\beta)}{g (\tan\alpha+\tan\beta)}$ where $a$ is an integer whose value is: