#coding:utf-8

def mame():
	n=raw_input().split()
	K=int(n[0])
	N=int(n[1])
	F=int(n[2])
	list=raw_input().split()
	list=map(int,list)

	f=0
	for i in list:
		f+=i

	mame=K*N

	if mame < f:	
		return -1
	else:
		for i in sorted(list):
			mame-=i
		return mame

if __name__=='__main__':
	print mame()