type punct=record
            x,y:longint;
            end;
     tablou=array [1..50002]of punct;
var t:tablou;
    i,m,n,k:longint;
    aria:extended;
    f,g:text;
    c:char;
    b:boolean;
    s:string;
begin
assign(f,'aria.in');
assign(g,'aria.out');
reset(f);
rewrite(g);
read(f,n);
for i:=1 to n do
 read(f,t[i].x,t[i].y);
aria:=0;
for i:=1 to n-1 do
   aria:=aria+(t[i].x*t[i+1].y-t[i+1].x*t[i].y)/2;
aria:=aria+(t[n].x*t[1].y-t[1].x*t[n].y)/2;
str(aria,s);
i:=1;
while s[i]<>'.'do
i:=i+1;
n:=i-1;
write(g,aria:n:5);
close(f);
close(g);
end.