Program a1;
type punct = record
      x,y:real;
      end;
var v:array[1..100002] of punct;
    arie,suma:real;
    n,i:longint;
    f,g:text;
begin
   assign(f,'aria.in');
   assign(g,'aria.out');
   reset(f);
   rewrite(g);
   read(f,n);
   suma:=0;
   for i:=1 to n do
      read(f,v[i].x,v[i].y);
   v[n+1] := v[1];
   for i:= 1 to n do
      suma:=suma+(v[n].x*v[n+1].y-v[n+1].x*v[n].y);
   suma := abs(suma);
   arie := suma/2;
   write(g,arie:0:5);
   close(f);
   close(g);
end.