Simple Console Calculator

Hello everyone,
I'm trying to make a simple calculator that doesn't care about priority and only do operation linearly.
Example: 4 + 5 * 2 = 18

Here's my code so far:

static void Main(string[] args)
        {
            int i = 0;
            float res;
            int p;
            int num2 = 0;
            int num1 = 0;

            if (int.TryParse(args[1], out p))
            {res = p;} // sets the res as the first number of the operation

            while (i < args.Length) // Doing the whole length of what we enterd
            {
                switch (args[i])
                    {
                        case "+" : // If it detects a +

                        if (int.TryParse(args[i+1], out p)) // Take the next argument
                        {
                            num2 = Int32.Parse(args[i+1]); // Make it an int
                            res += num2; // Add it to res (which is the 1st argument and the total of before)
                        }
                        break;

                        case "-":

                            if (int.TryParse(args[i + 1], out p))
                            {
                                num2 = Int32.Parse(args[i + 1]);
                                res -= num2;
                            }
                            break;

                        case "*":

                            if (int.TryParse(args[i + 1], out p))
                            {
                                num2 = Int32.Parse(args[i + 1]);
                                res *= num2;
                            }
                            break;
                            
                    } i++;
            }
            Console.WriteLine(res.ToString()); // Says unassigned res
            Console.ReadLine();
        }

However when I try it with a cmd, it crashes... Any ideas ?

Thanks in advance