use strict;
use warnings;
use v5.16;

my $KNF = <>;
# my $KNF = "10 10 3";
chomp $KNF;

my $line = <>;
# my $line = "10 20 40";
chomp $line;

my ( $K, $N, undef ) = split / /, $KNF;
my @years = split / /, $line;

# 家族の年齢の合計を求める
my $sum_years = 0;
map { $sum_years += $_ } @years;

# 豆の合計を求める
my $beans = $K * $N;

# 豆の総数から家族の合計年齢を引いた余りを求める
my $surplus = $beans - $sum_years;
if ( $surplus >= 0 ) {
    say $surplus;
}
else {
    say '-1';
}