# coding: utf-8

tsubu,fukuro,family = (int(x) for x in input().strip().split())
family_age = [int(x) for x in input().strip().split()]

family_sum = 0
family_sum = sum(family_age)

okashi_sum = tsubu * fukuro

if okashi_sum < family_sum:
    print("-1")
else:
    print(okashi_sum - family_sum)