program aria;
type punct=record
     x, y : double;
     end;
var i, n : longint;
    s: double;
    a : array[1..100000] of punct;
    fi, fo : text;
begin assign(fi, 'aria.in');
      assign(fo, 'aria.out');
      reset(fi);
      rewrite(fo);
      readln(fi, n);
      for i:=1 to n do readln(fi, a[i].x, a[i].y);
      close(fi);
      s:=0;     a[n+1]:=a[1];
      for i:=1 to n do s:=s+(a[i].x*a[i+1].y-a[i].y*a[i+1].x);
      write(fo,s/2:0:5);
      close(fo);
      end.