knfary = gets.split(' ').map(&:to_i)
aary = gets.split(' ').map(&:to_i)
sum = 0
sumfamily = 0
sum = knfary[0] * knfary[1]
aary.each do |i|
  sumfamily += i
end
zan = sum - sumfamily
if zan < 0
  puts -1
else
  puts zan
end