type punct=record
            x,y:extended;
            end;
     tablou=array [1..50002]of punct;
var t:tablou;
    i,n:longint;
    aria:extended;
begin
readln(n);
for i:=1 to n do
 read(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(aria/2:0:1);
end.