#include <algorithm>
#include <cstdio>
#include <iostream>
#include <map>
#include <set>
#include <string>
#include <vector>
#define rep(i, n) for (int i = 0; i < n; ++i)
#define FOR(i, a, b) for (int i = a; i < b + 1; ++i)
#define ll long long
#define pb(x) push_back(x)
using namespace std;

int main() {
  int k, n, f, a[100];
  cin >> k >> n >> f;
  rep(i, f) cin >> a[i];
  int cnt = k * n;
  rep(i, f) cnt -= a[i];
  cout << (cnt >= 0 ? cnt : -1) << endl;
  return 0;
}