Sunday's rook endgame, key idea

"Chess teaches you to control the initial excitement you feel when you see something that looks good and it trains you to think objectively when you're in trouble." - Stanley Kubrick

Sébastien - Wilfried

[Event "Ste Foy"] [Site "?"] [Date "2010.08.29"] [Round "?"] [White "Poulet, S."] [Black "Farge, W."] [Result "*"] [SetUp "1"] [FEN "8/5p2/2p1k3/r3P1p1/2PK2Pp/P4R1P/8/8 b - - 0 0"] *

Black to play, a pawn down. How to draw ?
(not very difficult)