if (@ARGV == 1) {
  $filename = shift;
} else {
  die "Usage: $0 filename\n";
}

$N = 1;
while (open TEST, "<", "$filename.$N") {
  $N += 1;
}

unless ($! =~ /No such file/) {
  die "Unexpected error opening $filename.$N: $!; aborting";
}

# $N is now the first number that *did not* work
$output_suffix = $N;
$input_suffix  = $N-1;

unless (open INPUT, "<", "$filename.$input_suffix") {
  die "Couldn't open $filename.$input_suffix for reading: $!; aborting";
}
unless (open OUTPUT, ">", "$filename.$output_suffix") {
  die "Couldn't open $filename.$output_suffix for writing: $!; aborting";
}

while (<INPUT>) {
  push @line, $_;
}

$i = 1;
while ($i < @line) {
  print OUTPUT $line[$i];
  $i += 2;
}
$i = 0;
while ($i < @line) {
  print OUTPUT $line[$i];
  $i += 2;
}