What kind of chalk do you feed garden snails?

[?? **~in la la land~**??]

I have 3 garden snails that i found somewhere near a swamp i heard that they eat chalk. Im just wondering if they eat regular chalk like the kind you use on the sidewalk or on the chalkboard or something else.
And im also wondering what the garden snails all time favorite food is and if my snails are really garden snails or just a swamp snail if there is such a thing.