#include <bits/stdc++.h>
#include <atcoder/modint>
using namespace std;
#define rep(i,n) for(int i = 0; i <n; i++)
#define all(a) a.begin(),a.end()
#define SORT(a) sort(all(a));
#define MIN(a) *min_element(all(a))
#define MAX(a) *max_element(all(a))
#define SUM(a) accumulate(all(a),0LL)
using ll=long long;
using ld=long double;
using ull=unsigned long;
ll MOD=998244353;

const ld PI=3.1415926535897932;
const int dx[]={0,1,0,-1,1,-1,1,-1};const int dy[]={1,0,-1,0,1,1,-1,-1};
const string ALPHA="ABCDEFGHIJKLMNOPQRSTUVWXYZ";const string alpha="abcdefghijklmnopqrstuvwxyz";
bool prime(ll n){
  for(ll i=2;i*i<n;i++){
    if(n%i==0){return false;}
  }
  return true;
}
using Graph = vector<vector<int>>;
ll fac[510000],finv[510000],inv[510000];
// 二項係数前
void comb_init(){
  fac[0]=fac[1]=1;
  finv[0]=finv[1]=1;
  inv[1]=1;
  for(ll i=2;i<510000;i++){
    fac[i]=fac[i-1]*i%MOD;
    inv[i]=MOD-inv[MOD%i]*(MOD/i)%MOD;
    finv[i]=finv[i-1]*inv[i]%MOD;
  }
}
ll comb(ll n,ll r){
  if(n<r)return 0;
  if(n<0||r<0)return 0;
  return fac[n] * (finv[r] * finv[n-r]%MOD)%MOD;
}
//二項係数終

int main(){
  cin.tie(nullptr);
  cout.tie(nullptr);
  ios::sync_with_stdio(false);
  ll K,N,F;cin>>K>>N>>F;
  ll ans=K*N;
  rep(i,F){
    ll A;cin>>A;
    ans-=A;
  }
  if(ans<0){cout<<-1<<endl;}
  else{cout<<ans<<endl;}
}