What should the Imam do if he says the Takbir for prayer with the intention of performing a different prayer?

Among the conditions for the validity of prayer is the intention. So, whoever says the takbir for a prayer and intends another prayer, that prayer is not valid for him due to lack of specification. He should interrupt it or intend to exit from it if he remembers that. If he was an imam, his prayer alone becomes invalid, while the prayer of the congregants is valid according to some scholars, because the reason for invalidity was hidden from them.