#include <bits/stdc++.h>
#define rep(i,n)for(long long i=0;i<(n);i++)
using namespace std;
typedef long long ll;
const int MOD=1e9+7;
const int MAX = 1000000;
const int INF = 1e9;
const double pi=acos(-1);
using namespace std;

int main(){
  ll k,n,f;
  cin >> k >> n >> f;
  ll sum=0;
  rep(i,f){
    ll a;
    cin >> a;
    sum+=a;
  }
  if(sum<=k*n)cout<<k*n-sum<<endl;
  else cout << -1 << endl;
  
  return 0;
}