my $num1 = <>;
my $num2 = <>;
chomp ($num1, $num2);

my ($beans, $bags, $people) = split (/ /, $num1);
my @num2 = split (/ /, $num2);

my $total_beans = $beans * $bags;

my $sum_age;

for my $age(@num2){
    $sum_age += $age;
}

if($total_beans - $sum_age < 0){
    print "-1\n";
}else{
    my $remainder = $total_beans - $sum_age;
    print "$remainder\n";
}