def check(a,b,x):
	if x < a or x > b:
		sys.exit()

import sys

list1 = input().split(' ')

K = int(list1[0])
N = int(list1[1])
F = int(list1[2])

check(1,1000,K)
check(1,1000,N)
check(1,100,F)

list2 = input().split(' ')

if len(list2) != F:
	sys.exit()

sum = K * N

for i in range(0,F):

	list2[i] = int(list2[i])
	check(1,100,list2[i])
	sum -= list2[i]

if sum >= 0:
	print(sum)
else:
	print(-1)