I have two field one is currency field(extra charge),other one is lookup field(Rate) , so i want to add these two field, that values stored in revised Rate(formula field).So directly update,when entering a form

'My Result is Rate is n 2000 and Extra Charge is  200 so result is   2200 '  but 

my result show like this :   2042688000000087568

I dont know how to solve it.

Please reply me.