/* -*- coding: utf-8 -*-
 *
 * 143.cc: No.143 豆 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

/* typedef */

/* global variables */

/* subroutines */

/* main */

int main() {
  int k, n, f;
  cin >> k >> n >> f;

  int asum = 0;
  for (int i = 0; i < f; i++) {
    int ai;
    cin >> ai;
    asum += ai;
  }

  int ans = k * n - asum;
  printf("%d\n", ans < 0 ? -1 : ans);
  return 0;
}