const max=100001;
type coordonate=record
x,y:real;
end;
vector=array[1..max] of coordonate;
var v:vector;
i,j,n:longint;
s:real;
f,g:text;
Begin
Assign(f,'aria.in');Reset(f);
Assign(g,'aria.out');Rewrite(g);
Readln(f,n);
For i:=1 to n do
With v[i] do
 Readln(f,x,y);
s:=0;
v[n+1].x:=v[1].x;
v[n+1].y:=v[1].y;
For i:=1 to n+1 do
s:=s+(v[i].x*v[i+1].y-v[i+1].x*v[i].y) ;
s:=abs(s/2);
Write(g,s:8:6);
close(f);close(g); end.