#include <bits/stdc++.h>
using namespace std;
int main(){
  int a,b,K;
  cin >> a >> b >> K;
  int value = a * b;
  for(int i = 0;i < K;i++){
    cin >> a;
    value -= a;
  }
  if(value < 0)cout << -1 << endl;
  else cout << value << endl;
  return 0;
}