Что значит "строгое чередование"?

It means that, for example, sequence
1 2 1 2 1 2 1
is valid, while sequence
1 1 2 1 2 1 1
is invalid.
More precise: if we put two different types of diploma into the same row, then no two diploma of the same type should be adjacent.

and total number of diplomas must be odd