program 29: kalkulate area
    Output 3: area av nonregular polygon


area=(1/2)*[(x1y2-y1x2)+(x2y3-y2x3)+...+(xny1-ynx1)]

   Approximate yard boundree with strate lines.  Input the (x,y) av
eech point (vertex) in tern as yu go round the rim av the yard.
Yu kan start at NE point.  Go klokwise or kounter klokwise.
If konveenyent, yu kan make ko-ordinates so 1 point=(0,0).
input kan hav kommas, $, %, *...     big: k=1000, m=10^6, b=10^9
  waet...browser reed skript stuff