var a:array [1..2,1..1000] of integer;
i,n:integer; s:real;

procedure lire;
var i:integer;
begin
  assign(input,'aria.in');
  reset(input);
  read(n);
  for i:=1 to n do
    read(a[1,i],a[2,i]);
  close(input);
end;

procedure ecrire;
begin
  assign(output,'aria.out');
  rewrite(output);
  write(s);
  close(output);
end;

begin
  lire;
  for i:=1 to n-1 do
    s:=s+(a[1,i]*a[2,i+1]-a[1,i+1]*a[2,i])/2;
  s:=s+(a[1,n]*a[2,1]-a[1,1]*a[2,n])/2;
  write(s);
end.