input_list = list(map(int,input().split()))
old_list = list(map(int,input().split()))

k = input_list[0]
n = input_list[1]
f = input_list[2]

need_bean = sum(old_list)
possible_bean = k * n

res = 0
if possible_bean < need_bean:
    res = -1
elif possible_bean >= need_bean:
    res = possible_bean - need_bean
    
print(res)