class Problem0143:
	def solve(this):
		k,n,f = map(int, input().split())
		a = list(map(int, input().split()))
		
		res = k*n - sum(a) 
		if res > 0:
			print(res)
		else:
			print(-1)
			
if __name__ =="__main__":
	problem = Problem0143()
	problem.solve()