class Beans
    attr_reader :k, :n, :f, :a

    def initialize(k: 0, n: 0, f: 0, a: [])
        @k, @n, @f, @a = k, n, f, a
    end

    def datainput
        begin
            @k, @n, @f = gets.chomp.split.map { |v| Integer(v) }
            @a = gets.chomp.split.map { |v| Integer(v) }
        rescue
            
        end
    end

    def remaining_beans(k, n, a)
        r = k * n - a.inject(:+)
        (r < 0)? -1 : r
    end

    def dataoutput
        puts remaining_beans(@k, @n, @a)
    end

    def run
        datainput
        dataoutput
    end
end

if $0 == __FILE__
    Beans.new.run
end