# coding: utf-8

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

fukuro_sum = kosu * fukuro
age_sum = sum(age)
ans = fukuro_sum - age_sum

if ans >=0:
    print(ans)
else:
    print("-1")