#include<bits/stdc++.h>
#define endl '\n'  // 使用标准换行符
#define rep(i,n) for(int i=0; i<(n); i++)
using namespace std;
using ll = long long;
using P = pair<int,int>;

#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#pragma GCC optimize("Ofast")

constexpr ll INF = 1e18;
constexpr int inf = 1e9;
constexpr double INFD = 1e100;
constexpr ll mod = 1000000007;
constexpr ll mod2 = 998244353;
const double PI = 3.1415926535897932384626433832795028841971;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  int K, N, F;
  cin >> K >> N >> F;
  int sum = 0;

  for(int i = 0; i < F; i++) {
    int a;
    cin >> a;
    sum += a;
  }

  if (sum > K * N) {
    cout << -1 << endl;
  } else {
    cout << K * N - sum << endl;
  }

  return 0;
}