I came across this formula in my textbook stating that when a body is projected vertically upwards and the time corresponding to height $H$ while ascending and descending are $t_1$ and $t_2$ respectively, when the velocity of projection is $\frac{g(t_1+t_2)}{2}$ where $g$ is acceleration due to gravity.

Can anyone please help me in deriving this formula? 
I know that in case of vertical motion upwards the initial velocity will be gt1. But I don't know how to proceed further.
Please help.