program Project2;

{$APPTYPE CONSOLE}
var i,n:integer;
s:real;
x,y:array[1..100] of real;
f1,f2:text;
begin
assign(f1,'input.txt');
reset(f1);
readln(f1,n);
assign(f2,'output.txt');
rewrite(f2);

for i:=1 to n do
readln(f1,x[i],y[i]);
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(f2,'s=',s:2:2);


end.