The Number Decoder

Problem Statement

The Number Decoder A simple decoder turns a number into a single uppercase letter of the English alphabet. The decoder works on the 26 letters A B C ... Z arranged in a ring, so that the letter after Z is A again. A pointer starts on Z (the position just before A ). For every unit of the number N , the pointer advances three letters forward around the ring. The letter the pointer lands on after all N advances is the decoded character. (inferred — the source lists only the six sample mappings 1 -> 'C' , 2 -> 'F' , 5 -> 'O' , 9 -> 'A' , 15 -> 'S' , 25 -> 'W' without spelling out the rule. An exhaustive search over every mapping of the form (a*N + b) mod 26 finds exactly one that reproduces all six, namely a step of three letters per unit starting just before A ; the rule above is that unique fit.) Given N , print the decoded character. Input Format A single line containing one integer N . Output Format A single line containing one uppercase English letter: the decoded r