#include <iostream>
#include <list>
#include <vector>
#include <string>
#include <stdio.h>
#include <stdint.h>
using namespace std;

using uint = unsigned int;
using ll = long long;

#define CIN( LL , A ) LL A; cin >> A 
#define GETLINE( A ) string A; getline( cin , A ) 


int main()
{

  CIN( ll , K );
  CIN( ll , N );
  CIN( ll , F );
  ll KN = K * N;
  ll A;

  for( ll i = 0 ; i < F ; i++ ){

    cin >> A;
    KN -= A;
    
  }

  if( KN < 0 ){

    KN = -1;

  }

  cout << KN << endl;
  return 0;

}