program aria;
var f,g:Text;
    x,y:array[0..100000] of longint;
    b1,b2:array[0..1 shl 23] of char;
    i,n,j,k,u,x1,y1,x2,y2:longint;
begin
assign(f,'aria.in');reset(F);settextbuf(f,b1);
assign(g,'aria.out');rewrite(G);settextbuf(g,b2);
 readln(f,n);
 for i:=1 to n do
        readln(f,x[i],y[i]);
 x[n+1]:=x[1];
 y[n+1]:=y[1];
 for i:=1 to n do
        k:=k+(x[i]*y[i+1])-(x[i+1]*y[i]);
 writeln(g,k div 2);
close(F);
close(G);
end.