Count of PersonId that have Flag A and B

Hello,

I want to know the number of people that have a common A and B as Flags, each person can have multiple As and/or multiple Bs (and also he can have As without B's and vice versa). This table shows some the data:

PersonID | Flag | Field1 | Field2 | Field3

12           | A     |

12           | A     |

13           | B     |

14           | B     |

15           | A     |

15           | B     |

16           | B     |

17           | A     |

12           | B     |

12           | A     |

The result of the count should be 2 because 12 and 15 are the only personID that have records with Flag A and B.

How to obtain this?