#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;

int main() {
  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(K*N < sum) cout << -1 << endl;
  else cout << K*N-sum << endl;
}