var n,i,j,k:longint;
    s:real;
    x,y:array[1..100000] of real;
    
function aria(x1,y1,x2,y2:real):real;
var k:real;
begin
  k:=x1*y2-x2*y1;
  writeln(k);
  aria:=k;
end;
    
begin
  assign(input,'aria.in');
  assign(output,'aria.out');
  reset(input);
  rewrite(output);
  read(n);
  for i:=1 to n do read(x[i],y[i]);
  s:=0;
  for i:=1 to n-1 do begin
    s:=s+aria(x[i],y[i],x[i+1],y[i+1]);
  end;
  s:=s+aria(x[n],y[n],x[1],y[1]);
  write(s/2:2:5);
end.