Task: Question Generation
Input (English): During the 1950s the rivalry was exacerbated further when there was a controversy surrounding the transfer of Alfredo di Stéfano, who finally played for Real Madrid and was key to their subsequent success. The 1960s saw the rivalry reach the European stage when they met twice in a controversial knock-out round of the European Cup, with Madrid receiving unfavourable treatment from the referee. In 2002, the European encounter between the clubs was dubbed the "Match of The Century" by Spanish media, and Madrid's win was watched by more than 500 million people.
Output (English): When did Barcelona meet Real Madrid twice in Europe?