program arie;
var f,g:text;
    n,i:longint;
    x,y:array[1..100002] of longint;
    s:longint;

begin
 assign (f,'aria.in'); reset (f);
 assign (g,'aria.out'); rewrite (g);
 readln (f,n);
 for i:=1 to n do
  readln (f,x[i],y[i]);
  x[n+1]:=x[1]; y[n+1]:=y[1];
  s:=0;
 for i:=1 to n do
  s:=s+(x[i]*y[i+1]-x[i+1]*y[i]);
 write (g,trunc(s/2));
 close (f); close (g);
end.