program aria;
  var n,i:longint;
      x,y,x1,y1,a,b,s:extended;
      buf:array[1..100000]of byte;

begin
  assign(input,'aria.in');
  reset(input);
  settextbuf(input,buf);
  assign(output,'aria.out');
  rewrite(output);
  readln(n);
  readln(a,b);
  x1:=a;y1:=b;
  for i:=2 to n do
    begin
      readln(x,y);
      s:=s+((abs(y)+abs(y1))*(x-x1)/2);
      x1:=x;y1:=x;
    end;
  x:=a;y:=b;
  s:=s+((y+y1)*(x-x1)/2);
  s:=abs(s);
  writeln(s:0:5);
  close(input);close(output);
end.