I really need some help to write a formula with nested functions.

How do I write a formula to calculate how many times a specific combination
of values appear in two different columns.

i.e. column A is populated with the values 1,2 and 3 in random order;
column B is populated with values a,b and c, also in random order. What
would the formula be to determine the number of times that the combination of
1 and a appear side by side on the same row; over a worksheet with 10 rows.

Thanks for any help in advance.