Case (i) both the letters are (T,T) or (S,S)= 2pairs

Case (ii) The letters may be any two from S,T,A,I,C = $^5C_2=10$ pairs

The order of the letters in the pair can be interchanged in 2 ways.

$\therefore $ The required no. of pairs $=2+2\times 10= 22$