I've been playing c5 as black in Queens Gambit Declined lately.
[FEN ""] 1. d4 d5 2. c4 e6 3. Nf3 Nf6 4. e3 c5
I just want to know, how bad an idea it is to play in this way? I saw someone play it in bullet game but I don't know when it's best to play.