#!/usr/local/bin/perl -w
#Author: Shraddha Kulkarni
use strict;

print "This script calculates area of the polygon enclosed in given points.\nEnter coordinates of points in clockwise sense.\n\n";

my @coords;
my $area=0;

while(1){
	print "Enter x coordinate: ";
	my $x=<STDIN>;
	last if ($x eq "\n");
	print "Enter y coordinate: ";
	my $y=<STDIN>;
	last if ($y eq "\n");
	print "Press Enter to get the area of polygon or Enter " if(@coords>=3);
	print "Next Point \n";
	push @coords,$x;
	push @coords,$y;
}
if(@coords<3){
print "A polygon has at least three points.. try again";
exit;
}

for (my $i=0;$i<@coords;$i=$i+2){
	my $j=$i+1;
	my $k=$i+2;
	my $l=$i+3;

	$j=$j-(@coords) if($j>=@coords);
	$k=$k-(@coords) if($k>=@coords);
	$l=$l-(@coords) if($l>=@coords);
	
	$area += ($coords[$k]*$coords[$j]-$coords[$i]*$coords[$l])/2; 
}
print "Area - $area";