program convexarie;
const nmax = 100005;
type coord = record
                x,y:real;
                end;
var f,g:text;
    n,i:longint;
    aria:real;
    v:array[1..nmax] of coord;
function det:real;
begin
        det:=v[i].x * v[i+1].y - v[i+1].x * v[i].y;
end;
begin
        assign(f,'aria.in'); reset(f);
        assign(g,'aria.out'); rewrite(g);
        readln(f,n);
        for i:=1 to n do
                readln(f,v[i].x,v[i].y);
        v[n+1]:=v[1]; aria:=0;
        for i:=1 to n do
                aria:=aria+det();
        writeln(g,aria/2:0:5);
        close(f); close(g);
end.