#coding: UTF-8

import sys
import re
import itertools
from math import log
from collections import deque

### defs ###


### main ###
K,N,F = map(int, sys.stdin.readline().split())
A = list(map(int,sys.stdin.readline().split()))

if (sum(A)>K*N):
	print(-1)
else:
	print(K*N-sum(A))