As most of you might know already, a prime number is an integer `n` with the following properties:

* it must be greater than 1
* it must be divisible only by itself and 1

And that's it: -15 or 8 are not primes, 5 or 97 are; pretty easy, isn't it?

Well, turns out that primes are not just a mere mathematical curiosity and are very important, for example, to allow a lot of cryptographic algorithms.

Being able to tell if a number is a prime or not is thus not such a trivial matter and doing it with some efficient algo is thus crucial.

There are already more or less efficient (or sloppy) katas asking you to find primes, but here I try to be even more zealous than other authors.

You will be given a preset array/list with the first few `primes`. And you must write a function that checks if a given number `n` is a prime looping through it and, possibly, expanding the array/list of known primes only if/when necessary (ie: as soon as you check for a **potential prime which is greater than a given threshold for each** `n`, stop).

#  Memoization

Storing precomputed results for later re-use is a very popular programming technique that you would better master soon and that is called [memoization](https://en.wikipedia.org/wiki/Memoization); while you have your wiki open, you might also wish to get some info about the [sieve of Eratosthenes](https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes) (one of the few good things I learnt as extra-curricular activity in middle grade) and, to be even more efficient, you might wish to consider [an interesting reading on searching from prime from a friend of mine](https://medium.com/@lcthornhill/why-does-the-6-iteration-method-work-for-testing-prime-numbers-ba6176f58082#.dppj0il3a) [she thought about an explanation all on her own after an evening event in which I discussed primality tests with my guys].

Yes, you will be checked even on that part. And you better be **very** efficient in your code if you hope to pass all the tests ;)

**Dedicated to my trainees that worked hard to improve their skills even on a holiday: good work guys!**

**Or should I say "girls" ;)? [Agata](https://www.codewars.com/users/aghh1504), [Ania](https://www.codewars.com/users/lisowska) [Dina](https://www.codewars.com/users/deedeeh) and (although definitely not one of my trainees) special mention to [NaN](https://www.codewars.com/users/nbeck)**

```python

USER_FILENAME = "<user_code>"
import sys, json, builtins
import math, heapq, bisect, itertools, functools, collections, statistics, fractions, decimal, operator, array, random, re, string, os, pathlib
try:
    import resource as _res
except Exception:
    _res = None
from collections import Counter, defaultdict, deque, OrderedDict
from functools import lru_cache, reduce
from itertools import accumulate, permutations, combinations, product
from math import gcd, ceil, floor, sqrt
from decimal import Decimal
try:
    from typing import *
except Exception:
    pass
try:
    sys.setrecursionlimit(1_000_000)
except Exception:
    pass
try:
    import site as _site
    try:
        _sys_sites = _site.getsitepackages() if hasattr(_site, "getsitepackages") else []
        for _p in _sys_sites:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
    try:
        _usp = _site.getusersitepackages()
        if isinstance(_usp, str):
            _usp = [_usp]
        for _p in _usp:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
except Exception:
    pass

def _disabled_input(*args, **kwargs):
    raise RuntimeError("input() is disabled in callable mode")
builtins.input = _disabled_input

ns = {"__name__": "__judge__"}  # prevent if __name__ == "__main__" blocks
for _k, _v in list(globals().items()):
    if not _k.startswith("_"):
        ns[_k] = _v
_CODE = 'primes = [2, 3, 5]\nlastCheck = [6]\n\ndef is_prime(x):\n    f = lastCheck[0]\n    if x <= f:\n        return x in primes\n    limit = x ** 0.5\n    for prime in primes:\n        if prime > limit:\n            return True\n        if x % prime == 0:\n            return False\n    isPrime = True\n    while f <= limit:\n        f1 = f + 1\n        if is_prime(f1):\n            primes.append(f1)\n        if x % f1 == 0:\n            isPrime = False\n        f5 = f + 5\n        if is_prime(f5):\n            primes.append(f5)\n        if x % f5 == 0:\n            isPrime = False\n        f += 6\n        if not isPrime:\n            break\n    lastCheck[0] = f\n    return isPrime'
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'is_prime'
if _fn_name:
    if "Solution" in ns:
        try:
            _obj = ns["Solution"]()
            if hasattr(_obj, _fn_name):
                _target = getattr(_obj, _fn_name)
        except Exception:
            _target = None
    if _target is None and _fn_name in ns and callable(ns[_fn_name]):
        _target = ns[_fn_name]
# Fallback: if there's exactly one user-defined function, use it
if _target is None:
    _cands = [v for k, v in ns.items() if callable(v) and getattr(v, "__module__", "") in ("__judge__", "__main__")]
    _user_funcs = [c for c in _cands if getattr(c, "__name__", "").startswith("_") is False]
    if len(_user_funcs) == 1:
        _target = _user_funcs[0]
if _target is None:
    raise RuntimeError(f"Could not resolve callable '{_fn_name}'")

_raw = sys.stdin.read().strip()
if _raw:
    _parsed = json.loads(_raw)
    if isinstance(_parsed, dict):
        _kwargs = _parsed.get("kwargs", {})
        _args = _parsed.get("args", [])
    else:
        _args, _kwargs = _parsed, {}
else:
    _args, _kwargs = [], {}
if not isinstance(_args, (list, tuple)):
    _args = [_args]
_res = _target(*_args, **_kwargs)
try:
    sys.stdout.write(repr(_res) + "\n")
except Exception:
    pass

```

Measured memory_bytes: 5686