title: Algorithm to find all convex quadrilaterals from the given list of 2d points
body: <p>I have to make a program to find all convex quadrilaterals from the given list of 2d points.
  I have tried it with vector cross product but it doesn't seem to be a correct solution. </p>
  
  <p>Maybe there is some effective algorithm to this problem but I can not find it.</p>
  
  <p>This is an example case with inputs and outputs:</p>
  
  <p><b>Input</b>
  <PRE>
  Number of Points:
  6
  </PRE>
  <PRE>
  coordinates of points (x,y):
  0 0
  0 1
  1 0
  1 1
  2 0
  2 1
  </PRE></p>
  
  <p><b>Output</b>
  <PRE>
  Number of convex quadrilaterals:
  9
  </PRE>
  </p>