Andrew and the Strings

Andrew likes strings very much. He has two strings A and B of N lower alphabet letters. We denote S[i, j] as the substring from ith to jth characters of string S. Andrew is interested in the number of such fours of integers (LA, RA, LB, RB), where 1 ≤ LA ≤ RA ≤ N, 1 ≤ LB ≤ RB ≤ N, and RA − LA = RB − LB, such that the Hamming distance between substrings A[LA, RA] and B[LB, RB] is not greater than