repeatedly increasing then decreasing

hi
this is probably simple but i am new to this, though am slowly getting to grips with it

i am trying to make a simple (ish) patch for a college assignment, i am using a metro to increase the speed of a sample which is playing in sfplay. it moves the speed from 0 up to 2x normal speed, then it goes straight back to 0 again.

my question is : how can i get it to slowly decrease the speed back down to zero when it reaches 2x instead of just going back to zero? and upon reaching zero to start the cycle again?

i know this is probably just simple maths and number boxes but i cant quite work it out

hope this makes sense

many thanks in advance