How to reduce frames of trajectory file?

GROMACS version:
GROMACS modification: Yes/No
Dear all

I want to decrease trajectory frames of my xtc file. All simulation run is 30 ns and I want to get trajectory every 30 ps . I know I have to use trajconv command but I don’t know what kind of options I have to use.

Thank you

gmx trjconv -skip

 -skip                 (1)
           Only write every nr-th frame

Thank you very much