Password Search

During World War II, the ability to send encoded messages was very important to the Allies. The messages were always sent after being encoded with a known password. Having a fixed password was insecure, so it was necessary to change it frequently. However, a mechanism for sending the new password was needed. One of the mathematicians on the cryptography team came up with an idea to send the password hidden within the message itself. The interesting part was that the recipient of the message only needed to know the length of the password and then search for it within the received message.

A password of length $n$ can be found by searching for the most frequent substring of $n$ characters within the text. After finding the password, all substrings that match the password are removed from the encoded message. Now, the password can be used to decode the message.

However, your task is simplified. You only need to write a program that, given the length of the password and the encoded message, finds the password according to the method described above.

Consider an example where the password length is n = 3 and the encoded message is $baababacb$. The password would be $aba$, because it is the substring of length $3$ that appears most frequently in the text (it appears twice). The other $6$ substrings ($baa$, $aab$, $bab$, $bac$, $acb$) each appear only once.