Exponent associates to the right 2^2^2^2^2^2 is about 6.03 * 10^19727 according to http://www.wolframalpha.com/input/?i=2^2^2^2^2^2 Reply
2^2^2^2^2^2 is about 6.03 * 10^19727 according to http://www.wolframalpha.com/input/?i=2^2^2^2^2^2