Save the Attendance

Problem description There are often situation at the college when people have to prove themselves to the teacher infront of the whole class. Once a boy named pankaj was the center of attack. The teacher gave him a string S consisting of alphabets ( a-z ) and dots ( . ) and asked him to enumerate the strings by filling the dots with alphabets from a-z, such that - abs(S[i] - S[i - 1]) ≤ d for i