Console.ReadLine() by itself wont work, as you need the value to be an integer. This is where our old friend Convert.ToInt32 comes in. This allows us to take the integer values out of the input and use it as a variable.
var Q = Console.ReadLine(Convert.ToInt32);
from your needs I suppose you need to try the eval() function...
var num1 = "123 + 456";
var sum = eval(num1);