type punct=record
            x,y:longint;
            end;
     tablou=array [1..100005]of punct;
var t:tablou;
    i,m,n:longint;
    aria:real;
    f,g:text;
    b:boolean;
    s:string;
begin
assign(f,'aria.in');
assign(g,'aria.out');
reset(f);
rewrite(g);
read(f,n);
for i:=1 to n do
 read(f,t[i].x,t[i].y);
aria:=0;
for i:=1 to n-1 do
   aria:=aria+(t[i].x*t[i+1].y-t[i+1].x*t[i].y)/2;
aria:=aria+(t[n].x*t[1].y-t[1].x*t[n].y)/2;
{str(aria:5:5,s);
while s[length(s)]='0' do
  delete(s,length(s),1);
if s[length(s)]='.'then delete(s,length(s),1);
write(g,s);}
write(g,aria:5:5);
close(f);
close(g);
end.