Math Girl's Annihilation Formula

The 13-year-old girl Filia was picked up from the ruins by the resistance organization and became a "tribune" to fight against the alien invasion. She used to just huddle in the corner of the classroom and use mathematics to escape everything. Now, amid countless destructions and losses, she picked up her pen - from "calculation" just to escape, to "calculation" to fight for protection. "I understand... This last question needs to be solved by me."