import sys
K,N,F=(int(x) for x in input().split())
A=(int(x) for x in input().split())

ans = K * N
for x in A:
	ans -= x
if ans < 0:
	ans = -1
if not 1 <= K <= 1000:
	ans = -1
if not 1 <= N <= 1000:
	ans = -1
if not 1 <= F <= 100:
	ans = -1
for x in A:
	if  not 1 <= x <= 100:
	 	ans = -1

print(ans)