Sum if two different values exist in same column for same id

[DMG]

I've a table of data where multiple rows exist for the same item, e.g.

Device     Value

1                 A
1                 B
2                 A
3                 A
3                 B

I want to sum the total number of situations where a unique device has rows of data that contain both A and B, so the result to the above data would be 2