10570: ABC122 —— D - We Like AGC
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Description
You are given an integer $N$. Find the number of strings of length $N$ that satisfy the following conditions, modulo $10^9+7$:
- The string does not contain characters other than `A`, `C`, `G` and `T`.
- The string does not contain `AGC` as a substring.
- The string does not contain characters other than `A`, `C`, `G` and `T`.
- The string does not contain `AGC` as a substring.
- The condition above cannot be violated by swapping two adjacent characters once.
Notes
A substring of a string $T$ is a string obtained by removing zero or more characters from the beginning and the end of $T$.
For example, the substrings of `ATCODER` include `TCO`, `AT`, `CODER`, `ATCODER` and (the empty string), but not `AC`.
Input
Input is given from Standard Input in the following format:
```
$N$
```
```
$N$
```
Output
Print the number of strings of length $N$ that satisfy the following conditions, modulo $10^9+7$.
Constraints
- $3 \leq N \leq 100$
Sample 1 Input
3
Sample 1 Output
61
There are $4^3=64$ strings of length 3 that do not contain characters other than A, C, G and T. Among them, only AGC, ACG and GAC violate the condition, so the answer is 64−3=61.
Sample 2 Input
4
Sample 2 Output
230
Sample 3 Input
100
Sample 3 Output
388130742