Hint 1: How many sequences end with '1' or '2'

Hint 2: s_a[0] = 1;         s_b[0] = 1;

Hint 3: s_a[i+j] = (s_a[i+j] + s_b[i])%inf;
Where i+j <=n and i<n and j<= a
 Repeat this with s_b

Good luck! :)