#include <iostream>
#include <algorithm>
#include <vector>
#include <numeric>

using namespace std;

int main(){
  int k, n, f, age, sum;
  cin>>k>>n>>f;
  vector<int> age_v;
  for (int i=0;i<f;i++){
    cin >> age;
    age_v.push_back(age);
  }
  sum = accumulate(age_v.begin(), age_v.end(), 0);
  if (k * n >= sum){
    cout << k * n - sum << endl;
  }else{
    cout << -1 << endl;
  }
}