Suppose we start from the back, there are 5 ways to put the last letter, 5 ways to put the last second, 4 ways for last third and then 1 way for the first letter, i.e., 5 X 5 X 4 X 3 X 2 X 1, which is the answer.
But if I start from the front, the answer comes out to be zero.
6 ways to arrange first letter, 5 for second, till 2 for fifth, now R cannot be the last letter, so there must be 0 ways to arrange last letter. 
Where am I wrong? Thank you.