program ploshchamnogokut;
{$APPTYPE CONSOLE}
var x,y:array[1..100] of integer;
i,n:integer;
s:real;
f1,f2:text;
begin
assign(f1,'input.txt');
reset(f1);
readln(f1,n);
for i:=1 to n do begin
readln(f1,x[i]);
readln(f1,y[i]);
end;
x[n+1]:=x[1]; y[n+1]:=y[1];
s:=0;
for i:=1 to n do begin
s:=s+(x[i]*y[i+1]-x[i+1]*y[i]);
s:=1/2*abs(s);
end;
assign(f2,'output.txt');
rewrite(f2);
writeln(f2,s);
close(f1);
close(f2);
end.