Question about Optical Goggles, HELP URGENT!?

I have a diopter of -3.25 on both my eyes (left & right).

i was shopping yesterday for OPTICAL SWIMMING GOGGLES. so i can see clearly when swimming, the only problem is that the prescription choices are:

1) Diopter -3.0
2) Diopter -3.5

Ok, so what number is equivalent to -3.25?

here is the link to the goggles…

http://www.campmor.com/outdoor/gear/Prod…

I HAVE A SWIMMING COMPETITION NEXT WEEK TO ATTEND TO…

http://www.campmor.com/outdoor/gear/Product___99321

here is the fixed link

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