Czech ACM Student Chapter

Czech Technical University in Prague

Charles University in Prague

Technical University of Ostrava

ˇ a

Slovak University of Technology

Pavol Jozef Saf´rik University in Koˇice

s

ˇ

University of Zilina

Masaryk University

Matej Bel University in Bansk´ Bystrica

a

University of West Bohemia

CTU Open Contest 2011

Text Encryption

encipher.c, encipher.C, encipher.java, encipher.p

To keep privacy of messages and prevent the aliens from reading them, we may use various

encryption algorithms. These algorithms encode a message into the so-called ciphertext that is

difficult (or impossible) to decode for anyone else than the intended recipient. Transposition

ciphers are a type of encryption that do not change the letters of the message but only change

their order ("shuffle" the letters). Of course, the shuffling must be reversible to allow later

decryption.

In this problem, we will consider a simple transposition cipher which shuffles the letters in

such a way that the decryption algorithm always takes every n-th letter. More specifically:

when decrypting, the first letter of the ciphertext is taken first, then the next n - 1 letters are

(repeatedly) skipped and the next letter taken, and so on until we reach the end of the ciphertext.

After that, we repeat the procedure starting with the second letter of the ciphertext, and so on

until all letters are used.

Your task is to implement the encryption algorithm for this cipher. For a given message, produce

the encrypted text (ciphertext). To make the cipher a little bit stronger, you should convert all

letters to uppercase and leave out all spaces between words.

Input Specification

The input contains several messages. Each message is described by two lines. The first line

contains one integer number N (1 ≤ N ≤ 1000). The second line contains the message. The

message will be at most 10 000 characters long, it will only contain letters and spaces, and there

will be at least one letter in each message.

The last message is followed by a line containing zero.

Output Specification

For each message, output the ciphertext that, after using the described decryption algorithm,

will result in the original message (with all spaces removed and all letters in uppercase).

Sample Input

2

CTU Open Programming Contest

7

This is a secret message that noone should ever see Lets encrypt it

15

text too short

0

Output for Sample Input

CMTMUIONPGECNOPNRTOEGSRTA

TESNUECHCAOLERIRGODLYSEENEEPITTEVTTSMHSESIAEAHRETSSTOSN

TEXTTOOSHORT