Excel Average if with multiple criteria

I am expanding on some help I got here yesterday.

I am using the 'average' and 'if' functions as below:

=AVERAGE(IF(YEAR(A:A)=1999,N:N))

I am wanting to also add the condition that if in column F the letters RD appear, the average will be calculated for that year.

Then, I want to calculate the average if the letters RD do NOT appear (i.e. exclude them).

I have tried the AND function but cannot get it to return any results.

TIA.