Primed for Action

Difficulty: Very easy
URL: https://app.hackthebox.com/challenges/867

Challenge Description
Intelligence units have intercepted a list of numbers. They seem to be used in a peculiar way: the adversary seems to be sending a list of numbers, most of which are garbage, but two of which are prime. These 2 prime numbers appear to form a key, which is obtained by multiplying the two. Your answer is the product of the two prime numbers. Find the key and help us solve the case.

The Challenge

Write a (Python, C, C++ or Rust) script that:

• Takes a single line of space-separated numbers as input.
• Filters out the prime numbers.
• Multiplies them all together.
• Prints only the resulting product — with no additional text, formatting, or debug output.

Solution Design

We approached this with a clean and structured layout.

Input Parsing

The input comes from stdin, typically using input() in Python. We expect a line like:

2 6 7 18 6

Prime Detection

We use a simple but efficient primality test.

This line defines the range of numbers used to check whether n is divisible by any odd number greater than 2 — which is part of a primality test.

Why start at 3?
We’ve already handled even numbers (specifically 2) earlier in the code. So we skip 2 and start checking from 3.

Why stop at int(n ** 0.5) + 1?
A number cannot be divisible by anything larger than its square root unless it’s also divisible by something smaller. Therefore, it’s enough to check up to and including the square root of n.

Since range() excludes the upper bound, we add +1 to ensure the square root itself is tested.

Why step by 2?
After 2, all even numbers can be skipped — they’re not prime, and they can’t be divisors of an odd number. This optimization cuts the number of checks in half.

Product Calculation

We multiply all primes using a basic loop and an accumulator.

Solution in Python

# Take input: a line of space-separated integers
input_line = input()

# Check whether a number is prime
def is_prime(n):
    if n < 2:
        return False
    if n == 2:
        return True
    if n % 2 == 0:
        return False
    for i in range(3, int(n ** 0.5) + 1, 2):
        if n % i == 0:
            return False
    return True

# Convert input to a list of integers
numbers = map(int, input_line.strip().split())

# Multiply all prime numbers found in the input
product = 1
for number in numbers:
    if is_prime(number):
        product *= number

# Output the result
print(product)