inpt = map(int, raw_input().split())
k = inpt[0]
n = inpt[1]
f = inpt[2]
age = map(int, raw_input().split())

bean = k * n
sum = 0
for i in range(0, f):
	sum += age[i]

if bean < sum:
	print -1
else:
	print bean - sum