var n,i,j:integer;
x,y:array[1..50000] of integer;
s:real;
begin
assign(input,'task10.in');
reset(input);
assign(output,'task10.out');
rewrite(output);
readln(n);
for i:=1 to n do
read(x[i],y[i]);
x[n+1]:=x[1];
y[n+1]:=y[1];
s:=0;
for i:=1 to n do
s:=s+(x[i]*y[i+1]-x[i+1]*y[i]);
s:=1/2*abs(s);
writeln(s:0:1);
close(input);
close(output);

end.