#include <iostream>
#include <vector>
#include <math.h>
#include <stdio.h>
#include <algorithm>
#include <string>
#include <map>
#include <queue>
#include <memory.h>
using namespace std;
const int INF = 2 << 27;
#define ll long long
typedef pair<int,int> P;

int main() {
  int k,n,f;
  cin >> k >> n >> f;
  int ans = k * n;
  for(int i = 0; i < f; i++) {
    int a;
    cin >> a;
    ans -= a;
  }
  cout << max(-1,ans) << endl;
}