Walks on a Triangle
combinatorics matrix-exponentiation
Three vertices of a triangle are all mutually connected. Count the closed walks of length N that start and end at a fixed vertex, modulo 109+7.
Input: one integer N.
Output: the count modulo 109+7.
Constraints:
- 1 ≤ N ≤ 1018
Sample Tests
Input 1
1
Output 1
0
Input 2
2
Output 2
2
Input 3
3
Output 3
2
Discussion (opens after you solve — no spoilers before)
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