k, n, f = gets.chomp.split(" ").map(&:to_i)
all_count = k * n
flag = 0
list = gets.chomp.split(" ").map(&:to_i)
for i in 0..f do
	all_count -= list[i].to_i
	flag = 1 if all_count < 0
end
puts flag == 0 ? all_count : -1