my $sum = 0;
$sum += (-1)**($_+1) * $_ for 1 .. 1000;
print "$sum\n";

##</code><code>##

my $sum = 0;
my $sign = 0;
for my $i (1 .. 1000) {
    $sum += ($sign ? -1 : 1) * $i;
    $sign = !$sign;
}
print $sum;

##</code><code>##

my $sum = 0;
my $sign = 0;
for my $i (1 .. 1000) {
    $sum += ($sign++ % 2 ? -1 : 1) * $i;
}
print $sum;

##</code><code>##

my $sum = 0.5*(1 + 999)*( (999-1)/2 + 1 ) - 0.5*(2 + 1000)*( (1000-2)/2 + 1 );
print $sum;