F = floor((current EXP) + ((current LVL)*50))
Of course, you do not have to use 50.
So let's say:
Lvl 1 needs 50 EXP to level up
Lvl 2 needs 150 EXP to level up
Lvl 3 needs 300 EXP to level up
Lvl 4 needs 500 EXP to level up
(I did those calculations in my head lol, might be wrong
)