import sys
sys.setrecursionlimit(100000)

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

a.sort(reverse=True)

aSum = 0
for i in a:
    aSum += i

if aSum > k * n:
    print(-1)
else:
    print(k*n - aSum)