Number of selections of four letters from the letters of the word ‘ASSASSINATION’

The letters are S,S,S,S; A,A,A,; N,N: I,I; T and O. When 4 distinct letters are selected, number of ways = 6C4=15 When two identical letters are selected, number of ways                    = 4C1×5C2=4×10=40 When two pair of identical letters are selected number of ways = 4C2=6 When there identical letters are selected and fourth is distinct, number of ways                   = 2C1×5C1=10 When all the four S ‘s are selected, then this is a unique way  ∴  Total number of selections =15+40+6+10+1=72