program Project1;
var i,n:integer;s:real;x,y:array [1..100] of real;f:text;
function L(x1,y1,x2,y2:real):real;
begin
L:=sqrt(sqr(x2-x1)+sqr(y2-y1));
end;
begin
assign(f,'input.dat');reset(f);
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];close(f);
assign(f,'output.sol');rewrite(f);
for i:=1 to n do s:=s+(x[i]*y[i+1]-x[i+1]*y[i]);
s:=abs(s)/2;
write(f,s:0:2);close(f);
end.