Calculați expresia 1-2+3-4+5-...

Program p1;
var s,i,n:integer;
Begin
s:=0;
write('Introdu n:');
readln(n);
for i:=1 to n do
if i mod 2=1 then s:=s+i
else s:=s-i;
writeln('Expresia=',s);
readln;
End.