Calculating Average Over Specific Period

I am attempting to calculate the avearge of many numbers over a user inputed amonut of time. The layout of the data is similar to:

Years.........Withdrawl
..1.............100,000..
..2.............100,000..
..3.............120,000..

The user enters the number of years. I need to compute the avearge for that specific amount of time. How can this be done???? Thanks.

Assuming the user inputs the number into cell D1 and your formula is in column C, try =SUMIF(A2:A3,"<="&D1,B2:B3)/D1