How to make a form which include two input fields and 2 fields PLUS,MINUS and a output field....when we insert the two inputs and click the plus button it automaticaaly calculate and transfer the result in output field?thanks in advance

How to make a form which include two input fields and 2 fields PLUS,MINUS and a output field....when we insert the two inputs and click the plus button it automaticaaly calculate and transfer the result in output field?

thanks in advance