Less than eight months after the release of Music of the Sun, Rihanna released her second album, A Girl Like Me, in April 2006. The album reached number five on the Billboard 200 and has been certified platinum by the RIAA, having shipped over one million units. The lead single, "SOS," peaked at number one on the Billboard Hot 100, becoming her first number-one in the United States. The second single, "Unfaithful," was a worldwide hit. In 2006, before the release, Rihanna served as an opening act for Gwen Stefani to promote the album. Rihanna then started touring with Pussycat Dolls from November to February 2007 in the United Kingdom.