Program p1;
var fi,fo:text; b1:array[0..1 shl 20] of char;
    x,y:array[0..100005] of real; s:real; i,n:longint;

begin
    assign(fi,'aria.in'); reset(fi);
    settextbuf(fi,b1); readln(fi,n); s:=0;
    assign(fo,'aria.out'); rewrite(fo);
    for i:=1 to n do readln(fi,x[i],y[i]); x[n+1]:=x[1]; y[n+1]:=y[1];

    for i:=1 to n do s:=s+x[i]*y[i+1]-x[i+1]*y[i];
    s:=s/2;
    writeln(fo,s:0:5);
    close(fi); close(fo);
end.