Sunderland have signed Charlton's Republic of Ireland midfielder Andy Reid in a deal which could cost up to £4million.The Black Cats earlier agreed a fee with Charlton and this evening completed the signing of the 25-year-old on a three-and-a-half-year deal. Sunderland boss Roy Keane told the club's official website, www.safc.com: 'Andy brings added quality to our side in an area that I felt needed to be strengthened.' Keane added: 'It's always healthy to have competition for places in the squad. 'I'm sure he'll do well for the club.' While Greg Halford earlier went the opposite way, on loan to Charlton, Reid becomes Keane's fifth signing this month. The former Nottingham Forest and Tottenham midfielder follows Jonny Evans, Jean-Yves Mvoto, Phil Bardsley and Rade Prica to the Stadium of Light. Reid earlier held talks with the Barclays Premier League side after the clubs reached agreement. Addicks boss Alan Pardew had told the club's website: 'A fee has been agreed and Andy Reid is talking to Sunderland. 'It was a difficult decision, but we have to look at ourselves and say `are we going to let this affect us or will this make us stronger'? Hopefully, the latter will happen.' Reid joined the Addicks from Tottenham in a move which could have amounted to £3million in August 2006, and has made 23 appearances so far this season. However, his campaign has been disrupted by injury and he has been sidelined with a knee problem since mid-December.