#!/usr/bin/awk

BEGIN {
	integral = 0;
	prev_x   = 0;
	prev_y   = 0;
	first_x  = -1;
	first_y  = -1;
}

{
	cur_x = $1;
	cur_y = $2;

	if (first_x == -1 && first_y == -1) {
		first_x = $1;
		first_y = $2;
		next;
	}

	integral += (cur_x - prev_x)*(cur_y + prev_y)/2;
	prev_x = cur_x;
	prev_y = cur_y;

}

END {
	integral -= (cur_x - first_x)*(cur_y + first_y)/2;
	print integral;
}