(K,N,F)=gets.chomp.split.map {|x| x.to_i}
ages=gets.chomp.split.map {|x| x.to_i}

def total_mame(k,n)
  return k * n
end

def eat(k,n,ages)
  ages_sum = ages.inject{|sum, i| sum + i}
  return -1 if ages_sum > k * n
  return k * n - ages_sum
end

puts eat(K,N,ages)