silver_Cowlendar — USACO January_Contest
Task type: batch | Time limit: 2 s | Memory limit: 256 MB | Difficulty: 17
Algorithms: number theory, pigeonhole principle, ad-hoc, mathematics, brute force

Bessie has woken up on a strange planet.  In this planet, there are $N$
($1\le N\le 10^4$) months, with $a_1, \ldots, a_N$ days, respectively
($1\leq a_i \leq 4 \cdot 10^9$, all $a_i$ are integers). In addition, on the
planet, there are also weeks, where each week is $L$ days, with $L$ being a
positive integer. Interestingly, Bessie knows the following:
For the correct $L$, each month is at least $4$ weeks long.
For
the correct $L$,  there are at most $3$ distinct values of $a_i\bmod L$.
Unfortunately, Bessie has forgotten what $L$ is! Help her by printing the sum of
all possible values of $L$.
Note that the large size of integers involved in this problem may require the
use of 64-bit integer data types (e.g., a "long long" in C/C++).
INPUT FORMAT (input arrives from the terminal / stdin):
The first line contains a single integer $N$. The second line contains $N$
space-separated integers, $a_1, \ldots, a_N$.
OUTPUT FORMAT (print output to the terminal / stdout):
A single integer, the sum of all possible values of $L$.
SAMPLE INPUT:
12
31 28 31 30 31 30 31 31 30 31 30 31
SAMPLE OUTPUT:
28
The possible values of $L$ are 1, 2, 3, 4, 5, 6, and 7.  For example, $L=7$ is
valid because each month is at least length $4 \cdot 7 = 28$ days long, and each
month is either 0, 2, or 3 mod 7.
SAMPLE INPUT:
4
31 35 28 29
SAMPLE OUTPUT:
23
The possible values of $L$ are 1, 2, 3, 4, 6, and 7. For example, $L=6$ is valid
because each month is at least length $4 \cdot 6 = 24$ days long, and each month
is either 1, 4, or 5 mod 6.
SCORING:
Inputs 3-4: $1 \leq a_i \leq 10^6$
Inputs 5-14: No additional
constraints
Problem credits: Brandon Wang

Starter code: