The next letter will be a "b." The pattern 2-1-3-2-4-3-etc. So
the "a"s and "b"s will each increase by one. First there were two
"b"s, then one "a." Then there were three "b"s and two "a"s. So
next there will be four "b"s and three "a"s, five "b"s and four
"a"s, and so on.