#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cmath>
#include <iostream>
#include <queue>
#include <list>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;


int main(){
  int K, N, F;
  int sum = 0, x;
  cin >> K >> N >> F;
  rep(i,F){
    cin >> x;
    sum += x;
  }

  if(K*N<sum){
    cout << -1 << endl;
  }else{
    cout << K*N-sum << endl;
  }
  return 0;
}