Statement

To better protect the server containing the future tasks of this competition, Luca is choosing a long passphrase to encrypt the tasks archive. Due to the current pandemic restrictions, Luca is unable to meet the rest of the team and directly communicate to them a newly chosen passphrase: he is left with no other option than sending it using a potentially insecure channel, and for this reason he must make it unrecognizable to external malicious eyes. After a careful evaluation of potential attackers, Luca devised an infallible plan to safely communicate the passphrase: he is going to remove exactly K characters from the sequence of passphrases used by the team so far, joined together. These combined passphrases form a string of length N that everybody in the team knows; he can then only communicate the removed characters, making the new passphrase unintelligible to others while still being reconstructible by his colleagues...