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use strict;
use warnings;
# calc GCD Euclid's algorithm
# https://en.wikipedia.org/wiki/Greatest_common_divisor#Euclid's_algorithm
sub gcd {
my ($x, $y) = @_;
$y ? gcd($y, $x % $y) : $x;
}
# sum GCD of all possible unique pairs between 1 and $N
sub sum_gcd_unique_pairs {
my $N = shift;
my $sum = 0;
for my $i (1 .. $N) {
for my $j ($i+1 .. $N) {
$sum += gcd($i, $j);
}
}
return $sum;
}
1970;
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