At the Tournament of Progressive Combos, better known as TOPC, fighters compete to land the legendary TOPC Crit.
Each attack combo produces three damage values: $$$a$$$, $$$b$$$, and $$$c$$$. Three values form an arithmetic progression if the difference between every pair of consecutive values is the same. In other words, they must satisfy $$$$$$ b-a=c-b. $$$$$$ This shared difference is called the common difference.
A combo triggers a TOPC Crit if and only if its three damage values form an arithmetic progression in the given order. When it does, their common difference $$$x$$$ is known as the secret.
Find this value. It is, indeed, the game's top secret.
The only line contains three integers $$$a$$$, $$$b$$$, and $$$c$$$.
- $$$-10^9 \le a,b,c \le 10^9$$$
If $$$a$$$, $$$b$$$, and $$$c$$$ form an arithmetic progression, output secret $$$x$$$, where $$$x$$$ is the common difference.
Otherwise, output not secret.
1 4 7
secret 3
1 2 4
not secret
Explanation for Sample 1
The two consecutive differences are both $$$3$$$, so the three integers form an arithmetic progression with common difference $$$3$$$.
Explanation for Sample 2
The consecutive differences are $$$1$$$ and $$$2$$$, so the three integers do not form an arithmetic progression.
