# -*- coding:utf-8 -*-

if __name__ == "__main__":
	k, n, f = map(int,raw_input().split())
	age = map(int,raw_input().split())
	sum = 0
	for i in xrange(f):
		sum += age[i]
	if sum > k * n:
		print -1
	else:
		print k * n - sum