The problem is that "110101111101111000011010" isn't a valid base ten number - it's a valid binary number, but it's way, way bigger than the largest value that will fit in an integer: 2,147,483,647
So when you do this:
a=Convert.ToInt32(Console.ReadLine());
It tries to convert it as a decimal (base 10) number and overflows.
If you want it as a binary number, then use a overload:
a=Convert.ToInt32(Console.ReadLine(),2);
But do be aware that users make mistakes: and a single character wrong will crash your application.