wxwx, 3 bytes
It's W, with just one instruction added: square root. Turns out that this is very useful! (P.S. the built-in was added before the challenge.)
2Q/
Explanation
2Q % Find the square root of 2
a / % Divide the input by it
% If one operand is an integer,
% the program will automatically
% try to trunctuate to an integer