#include <iostream>
#include <string>
#include <cctype>
#include <typeinfo>
#include <cstdlib>
#include <vector>
#include <algorithm>

using namespace std;

int main() {
  int k, n, f;

  cin >> k >> n >> f;
  int sum = 0;

  for(int i = 0;i<f;i++){
    int tmp;
    cin >> tmp;
    sum = sum + tmp;
  }


  if(k*n >= sum){
    cout << k*n-sum << endl;
  }else{
    cout << "-1" << endl;
  }
  return 0;
}