#include "bits/stdc++.h"
using namespace std;

int main() {
  int K, N, F; cin >> K >> N >> F;
  int beans = K * N;
  int A;

  for(int i = 0; i < F; ++i) {
    cin >> A;
    beans -= A;
  }
  if(beans < 0) cout << -1 << endl;
  else cout << beans << endl;
}