uses crt;
var a: array[1...100] of integer;
i,n,hh: longint;
begin
clrscr;
write ('nhap so phan tu: '); readln (n);
for i:=1 to n
begin
write ('nhap a[',i,']: '); readln (a[i]);
if i mod 2=0 then hh:=hh-a[i];
end;
writeln ('hieu la: ',hh);
readln;
end.