Hi guys. I realised that Facebook hacker cup 2021 qualification round is coming soon, so I decided to practice using past year problems. I started with the 2020 qualification round. I finished solving problem A: Travel Restrictions. I followed the instructions from facebook's hacker cup team. I downloaded the input file and printed the output to an output file named output.txt. Then I uploaded the output and my source code to the grading server but an error message appeared, saying "Oops, an error occurred on our end while processing your submission. Please try again."

Does anyone know how to fix this. Please help, I really want to verify whether my solution is right. Thanks.