Help Reading a Knitting Pattern

Hi,
I am finishing a sweater and am stuck on a sleeve instruction. It says:
Increase 1 st. at each end of every 4th row 13 times, then 1 st. alt. on every 2nd and 4th row 18 times - 107(115) sts.
The motif pattern it uses is 8 total rows (four pattern "front" rows and four backside rows). My question is, do I first increase one stitch on rows 4 and 8 13 times? Then what does the "1 st. alt. on every 2nd and 4th row 18 times" mean?
Thanks for your help. It is most appreciated!!!

Yes, first do the increase at each end of 4, 8, 12, 16 ...

That'll keep you going while the rest of us work out what the other bit means! To start with I'd be working out how many stitches I had at that point to figure out what would get me to the end stitch number. Some patterns are hard to figure.

How many stitches have you before you start the first increase?

I think I would ignore the alt & just increase on every 2nd & 4th row to 107 or 115 sts

I think the alt must mean that each time you increase it should be at a different end of the row so the increases are the same on each side. But if your maths is good you could work out the difference between the number of stitches you have at the end of the first lot of increases and the 107 or 115 that you are aiming for. That will give the number of rows and therefore how often to increase. Sorry that may be clear as mud;in which case ignore!

How many stitches do you have BEFORE any increases?
I think it means increase every 4th row for a while, then every 2nd and 4th row, ie twice as fast an increase. That way you'ld have a slow, long increase followed by a more acute one which would give you more room in the upper arm.
The maths may save you!
PS I think the pattern is not very clearly written, it's not you

If you read it as ' increase at one end of the 2nd row and then at the other alternate end of the 4th row so the increases balance' this would be the same rate of increase as the first part- which is pretty clearly written.
I think this is not the right answer.
The stitches before any increases are X. Then you were to increase 13 times, at 2 sts a time = 26 more stitches. So your sleeve is at this stage (X + 26)
You are to wind up with 107 or 115 stitches. Call this number Y
If you subtract 26, and then subtract X from that number you will know how many you have to increase.:confused:
Y-26 = (something)
(something) - X = how many more stitches you have to increase.
Remember the increases will be in 2's so halve that number for the increase rows.
Alternately, throw the knitting across the room, scream a bit and go and talk to the goats.:eek::eek:

How did you go RJG? Is the sleeve finished?

I always knit two sleeves together then they look the same even if you have made a small mistake in your counting!