Background
Problem 9A on the 2016 Euclid was stated as follows:
The string AAABBBAABB
is a string of ten letters, each of which is A
or B
,
that does not include the consecutive letters ABBA
.
The string AAABBAAABB
is a string of ten letters, each of which is A
or B
,
that does include the consecutive letters ABBA
.
Determine, with justification, the total number of strings of ten letters, each of
which is A
or B
, that do not include the consecutive letters ABBA
.
Problem Statement
Since computers are pretty fast, we've decided to replace ten with .
Determine, without justification, the total number of strings of letters, each of
which is A
or B
, that do not include the consecutive letters ABBA
, modulo .
Constraints
Input Specification
A single integer, .
Output Specification
A single integer, the answer.
Sample Input 1
4
Sample Output 1
15
Sample Input 2
10
Sample Output 2
631
Sample Input 3
123123123
Sample Output 3
952227253
Sample Explanation 1
Out of the possible strings of length , one of them contains the consecutive letters ABBA
. Therefore, of the strings are valid.
Comments