program aria;
var x,y:array[0..100005] of longint;
    n,i:longint;
    s:extended;
    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 readln(f,x[i],y[i]);
 x[n+1]:=x[1]; y[n+1]:=y[1];
  for i:=2 to n+1 do s:=s+x[i-1]*y[i]-x[i]*y[i-1];
  s:=s/2;
  writeln(g,s:0:0);
  close(f); close(g);
end.