k, n, f = list(map(int, input().split()))
a = list(map(int, input().split()))

def sum_list(l):
    s = 0
    for i in range(len(l)):
        s += l[i]    
    return s

soy_sum = k * n
age_sum = sum_list(a)
if soy_sum < age_sum:
    print("-1")
else:
    remnant = soy_sum - age_sum
    print("{}".format(remnant))