Consider the array formula :
=SUM((TRIM(MID(SUBSTITUTE(A1,";",REPT(" ",255)),1+(ROW(A1:A999)-1)*255,255)) & "0")/10)
Array formulas are entered with CNTRL-SHFT-ENTER rather than just the ENTER key.
So if A1 contained:
1;2;3
the formula should return 6