Kata

Let's look at the number 12343.

This number differs by 1 in all adjacent digits. This number is called the 'Stair number'.

When number's length is given, write a function to find out how many Stair numbers are in total length. (Testcases are never start with zero.)

Returns the remainder of the answer divided by 1000000007 (1e9 + 7)

/Input/

1 <= n <= 1000 // n is integer.

/Example Output/

solution(1) returns 9.
solution(2) returns 17.