%!PS-Adobe-3.0 EPSF-3.0 
    %%DocumentMedia: a4 595 842 80 () ()
    newpath
    300 500 moveto
    /poly {
            /side exch def
            /angle  360 side div def
           side {
           angle rotate
           720 side div  0 rlineto
            } repeat
            stroke
            closepath 
    } def
    7 poly
    showpage

Save it as figure.ps. Also, you can try "8 poly", or "1000 poly" above "showpage".