program aria;
var
f1,f2:text;
x,y:array[0..100005] of real;
i,n:longint;
k:extended;
begin
assign(f1,'aria.in');reset(f1);
assign(f2,'aria.out');rewrite(f2);
readln(f1,n);
for i:=1 to n do
readln(f1,x[i],y[i]);
x[n+1]:=x[1];
y[n+1]:=y[1]; K:=0;
for i:=1 to n do
k:=k+(x[i]*y[i+1])-(x[i+1]*y[i]);
writeln(f2,abs((k / 2)):0:5);
close(f1);
close(f2);
end.