program task10;

{$APPTYPE CONSOLE}

var x,y:array[0..50001] of integer;
    i,j,n:integer;
    s:real;
begin
assign(input,'task10.in');
reset(input);
readln(n);
for i:=1 to n do read(x[i],y[i]);
close(input);

x[n+1]:=x[1];
y[n+1]:=y[1];
s:=0;
for i:=1 to n do begin
  s:=s+(x[i]*y[i+1]-x[i+1]*y[i]);
end;
s:=1/2*abs(s);

assign(output,'task10.out');
rewrite(output);
writeln(s:0:1);
close(output);
end.