a six digit no. is to be formed using the digits 1,2,3,4,5,6 such that 6 should never come just before 5 , 5 should never come just before 4 , 4 should never come just before 3 , 3 should never come just before 2 , 2 should never come just before 1. find the total no, of permutations

Co-incididentally it''s the same problem i was dealing with for last few days. Actually its one of the toughest problems i have ever faced. it will take pages and pages if i start explaining it. So i will just give the answer. It will require a general term with recurrence. The answer for 6 digits is 309.