Eugene and function

Eugene loves sequences, especially arithmetic progressions. One day he was asked to solve a difficult problem. If a sequence of numbers A1, A2, ... , AN form an arithmetic progression A, he was asked to calculate sum of F(Ai), for L ≤ i ≤ R. F(X) is defined as: If X F(X) = X. Else F(X) = F(sum_of_digits(X)). Example: F(1378) = F(1+3+7+8) = F(19) = F(1 + 9) = F(10) = F(1+0) = F(1) = 1 I