#include <algorithm>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>

using namespace std;

typedef long long ll;

#define MOD 1000000007

int main() {
  int k, n, f;
  cin >> k >> n >> f;
  vector<int> a(f);
  for (int i = 0; i < f; ++i) {
    cin >> a[i];
  }
  int sum = 0;
  for (int i = 0; i < f; ++i) {
    sum += a[i];
  }
  int ans = k * n - sum;
  cout << (ans < 0 ? -1 : ans) << endl;
  return 0;
}