You could also use SUMIF. At the end of your table of data, use the formula
=SUMIF($A$1:$A$20,2001,$C$1:$C$20)
Where your 'year' data is in column A, and frequency is in column C.
This will then total all the frequency items that appear in a row with 2001. Copy the formula for each other year you're interested in.
To make it neater, instead of typing '2001' in the above formula, type 2001 (2002, 2003 etc) in cells, and reference the cells in your formula.