Shortest code possible

ferrante.js

var alfabet="abcdefghjpz679";
var convertToLetters = function(n){for(var a=alfabet,c='',l=a.length;c=a[n%l]+c,n=n/l|0,n--;);return c};
var convertToNumber = function(s,i,v){with(alfabet)for(i=v=0;s[i];v=v*length-~indexOf(s[i++]));return~-v};