s = gets.split.map(&:to_i)
sn = s[0] * s[1]
f = s[2]
a = gets.split.map(&:to_i)
f.times do |i|
    sn -= a[i]
    if sn < 0 
        break
    end
end
if sn >= 0
    puts sn
else
    puts -1
end