type punct=record
            x,y:longint;
            end;
     tablou=array [1..1000002]of punct;
var t:tablou;
    i,n:longint;
    aria:extended;
    f,g:text;
begin
assign(f,'aria.in');
assign(g,'aria.out');
reset(f);
rewrite(g);
readln(f,n);
for i:=1 to n do
 read(f,t[i].x,t[i].y);
n:=n+1;
t[n].x:=t[1].x;
t[n].y:=t[1].y;
aria:=0;
for i:=1 to n do
   aria:=aria+(t[i].x*t[i+1].y-t[i+1].x*t[i].y);
write(g,aria/2:0:5);
close(f);
close(g);
end.