use strict;
use warnings;

my $num = <STDIN>;
my @w = split(' ', <STDIN>);

my $half = $num % 2 == 0 ? $num/2-1 : $#w/2;
my @ary_a = sort {$a <=> $b} @w[0..$half];
my @ary_b = sort {$a <=> $b} @w[$half+1..$#w];

my @sum;
my %store;

for(;;){
  @sum = (0, 0);
  $sum[0] = eval join('+', @ary_a);
  $sum[1] = eval join('+', @ary_b);

  if(defined($store{$sum[0]}) and defined($store{$sum[1]})){
    print 'impossible';
    last;
  }else{
    $store{$sum[0]} = 1;
    $store{$sum[1]} = 1;
  }

  if($sum[0] > $sum[1]){
    push @ary_b, shift @ary_a;
    @ary_b = sort {$a <=> $b} @ary_b;
  }elsif($sum[0] < $sum[1]){
    push @ary_a, shift @ary_b;
    @ary_a = sort {$a <=> $b} @ary_a;
  }else{
    print 'possible';
    last;
  }
}