How can you calculate with a String in code?

Hey,

So as usual we don't really have much to do right now so we started multiply one by two until we could no more. The only problem is that we don't know if our answers are right after a certain number since our calculators start showing the e+01 at the end which makes to number invalid to check our answers.

I know that it is possible to calculate with a string (bundle of characters not what girls wear :P) but now my script says that 16*2 = 122 which is definitely not right but when you split it into 12 and 2 you only need to do the 2 * 10 to get 20 and add the 12 to get 32.

This is my current script in C# (but any code will do)

        string last = "1";
        int m = 2;

        while (true)
        {
            String newStr = "";
            int nextAdd = 0;
            foreach (char c in last)
            {
                int n = int.Parse(c.ToString());
                n = (n+nextAdd) * m;
                if (n < 10)
                {
                    nextAdd = 0;
                }
                while (n > 10)
                {
                    //Console.WriteLine(n);
                    n -= 10;
                    nextAdd++;
                }

                //Console.WriteLine(n.ToString());
                newStr = n.ToString() + newStr;

                //Console.WriteLine(newStr);
            }
            if (nextAdd != 0)
            {
                newStr = nextAdd.ToString() + newStr;
            }

            Console.WriteLine(newStr);
            last = newStr;

            Thread.Sleep(500);
        }

What am I doing wrong?