Calculator Program in Progress

function calculator()
    while true do
        print("What is the operation you want to act upon?")
        op = read()
        if op == "add" or "subtract" or "multiply" or "divide" then
            print("What is the first number acted upon?")
            num1 = tonumber(read())
            print("and the second number?")
            num2 = tonumber(read())
            if op == "add" then
                result = num1 + num2
                print(result)
            elseif op == "subtract" then
                result = num1 - num2
                print(result)
            elseif op == "multiply" then
                result = num1 * num2
                print(result)
            elseif op == "divide" then
                result = num1 / num2
            end
        elseif op == "exit" then
            print("Are you sure you want to exit the calculator application?")
            confirm = read()
            if confirm == "yes" then
                break
            end
        end
    end
end