If you want to calculate values in JavaScript, you can use the mathematical operators addition ("+"), subtraction ("-"), multiplication ("*") and division ("/").
Example:
var result = jr_get_value(“amount“) * jr_get_value("price“); // multiplication
var result = jr_get_value(“external_participants“) + jr_get_value("internal_participants“); // addition
var result = jr_get_value(“participants“) - jr_get_value("external_participants“); // subtraction
var result = jr_get_value(“price“) / jr_get_value("participant“); // division
