program aria;
const maxn=100005;
type punct=record
                x,y:real;
                end;
var p:array[1..maxn] of punct;
    s:real;
    n,i:longint;

function Cross_Product(a,b:punct):real;
begin
 Cross_Product:=a.x*b.y-a.y*b.x;
end;

begin
assign(input,'aria.in');
reset(input);
assign(output,'aria.out');
rewrite(output);

 readln(n);

 for i:=1 to n do
      readln(p[i].x,p[i].y);

 s:=0;
 p[n+1]:=p[1];
 for i:=1 to  n do
      s:=s+cross_product(p[i],p[i+1]);
 s:=s/2;

 writeln(s:0:5);

 close(output);
end.