I agree this is far from easy. As we progress in this series the difference in difficulty between the projecteuler.net problem and the hackerrank problem becomes vast. I can solve the first version easily using simple methods and then sweat for days or weeks to meet the performance constraints of the latter, often with a completely different solution approach. This one for example nearly made me quit.

I lost my confidence because this is marked as easy.

Hints for those who come later:

• Your clever DP solution will score 0.00
• Your O(n) solution will score 0.00
• I got 100 after looking up some basic modular arithmetic and combining everything