string to ints

        Console.WriteLine("Please enter a four digit number that does not begin with a zero");
        string nStr;

        do
        {
	        nStr = Console.ReadLine();
            if (nStr.Length != 4 || !Regex.IsMatch(nStr, @"^\d+$") || Regex.IsMatch(nStr, @"^0")) //Ask for a correct number until all three conditions are met
            {
                Console.WriteLine("Please enter a four digit number that does not begin with a zero");
            }
            else
            {
                break;
            }
	    } while (true);

        char[] nArr = nStr.ToCharArray();
        int a = int.Parse(nStr[0].ToString());
        int b = int.Parse(nStr[1].ToString());
        int c = int.Parse(nStr[2].ToString());
        int d = int.Parse(nStr[3].ToString());