#include <bits/stdc++.h>

using namespace std;

const int INF = 1 << 30;

int main()
{
  int K, N, F;
  cin >> K >> N >> F;
  K *= N;
  while(F--) {
    int A;
    cin >> A;
    K -= A;
  }
  if(K < 0) cout << -1 << endl;
  else cout << K << endl;
}