I'm a bit stuck on a situation. I could greatly use some help. I have been keeping track of NBA scores in excel. Let's just use the chicago bulls in this example. I am trying to find the average amount of points that the bulls win by, on only "won" games (not games they've lost)

I have 4 columns labeled as follows: | W/L | Pts | Opponent Pts | Difference |
Each row will contain data from 1 game. 10 rows would have scores for 10 games.

When the amount of points the bulls score are greater than their opponent's score, a "W" will appear in the first column. And when they score less, a "L" will appear instead. (I've written the function for this already)

The points scored will be calculated, and difference is shown in the fourth column. (I've written the function for this already)

Now here's what I'd like to do...
I'd like a formula that will give me the average of all the data in the fourth column, with the criteria that only if the first column contains a "W"

Could someone explain the function i would need to write in order to achieve this?