Bebras 2021 - Level 2 Question 11

To add two letters, the beavers add their positions in the alphabet. The result is the letter corresponding to the sum.



| Letter | A | B | C | D | E | F | G | H | I | J | K | L | M |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| Value | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 |

| Letter | N | O | P | Q | R | S | T | U | V | W | X | Y | Z |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| Value | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 |

For example: $A+A=B$; $A+B=C$. If the result is greater than 26, counting continues from A. For example: $Z+B=B$; $Y+C=B$.

Beaver Lenka uses this rule to encode messages:

- Choose a random number and call it the **key**.
- Write the message on one line, then write it again on the next line, shifted to the right by the number of positions specified by the key.
- Add the letters in each column to obtain the encoded result.

For example, encoding TEA with key 1 gives TYFA.



What does Lenka obtain by encoding **COMPUTER** with key 3?