#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); i++)
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<ll> vll;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<vector<ll>> vvll;
const ll inf = 1e16;
const ll md = 1000000007;

int main() {
  int k,n,f;
  cin>>k>>n>>f;
  vi a(f);
  rep(i,f) cin>>a[i];
  int s=0;
  rep(i,f) s+=a[i];
  int ans=k*n-s;
  if(ans<0) puts("-1");
  else cout<<ans<<endl;
  return 0;
}