Problem J4: Big Bang Secrets

Problem Description

Sheldon and Leonard are physicists who are fixated on the BIG BANG theory. In order to exchange secret insights they have devised a code that encodes UPPERCASE words by shifting their letters forward.

Shifting a letter by $S$ positions means to go forward $S$ letters in the alphabet. For example, shifting B by $S=3$ positions gives E. However, sometimes this makes us go past Z, the last letter of the alphabet. Whenever this happens we wrap around, treating A as the letter that follows Z. For example, shifting Z by $S=2$ positions gives B.

Sheldon and Leonard's code depends on a parameter $K$ and also varies depending on the position of each letter in the word. For the letter at position $P$ , they use the shift value of $S = 3P + K$ .

For example, here is how ZOOM is encoded when $K = 3$ . The first letter Z has a shift value of $S = 3\times 1 + 3 = 6$ ; it wraps around and becomes the letter F. The second letter, O, has $S = 3 \times2 +3 = 9$ and becomes X. The last two letters become A and B. So Sheldon sends Leonard the secret message: FXAB

Write a program for Leonard that will decode messages sent by Sheldon.

Input Specification

The input will be two lines. The first line will contain the positive integer $K$ ( $K \lt 10$ ), which is used to compute the shift value. The second line of input will be the word, which will be a sequence of uppercase characters of length at most 20.