#include <bits/stdc++.h>
#include<atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long; // -2^63 ~ 2^63-1 (9.2*10^18)
using ull = unsigned long long; // 0 ~ 2^64-1 (1.8*10^19)
using ld = long double;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep2(i, m, n) for (int i = (m); (i) < int (n); i++)
#define all(v) v.begin(), v.end()
#define bit(n) (1ll<<(n)) // 2^n
#define sz(x) ((int)(x).size())
#define fi first
#define se second
template<class T> using maxpq = priority_queue<T>;
template<class T> using minpq = priority_queue<T, vector<T>, greater<T>>;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
const ll llINF = 1000000000000000000;
void YesNo(bool flag){
    if(flag) cout << "Yes" << endl;
    else cout << "No" << endl;
}
// 多次元vector生成
template<ll idx = 0, ll n, class T>
auto make_vec(const ll (&d)[n], T&& init) noexcept {
    if constexpr (idx < n) return std::vector(d[idx], make_vec<idx + 1>(d, std::forward<T>(init)));
    else return init;
}
template<class T, ll idx = 0, ll n>
auto make_vec(const ll (&d)[n], const T& init = {}) noexcept {
    if constexpr (idx < n) return std::vector(d[idx], make_vec<idx + 1>(d, init));
    else return init;
}
// auto 変数名 = make_vec<型>({a, b, ...}, 初期値);

int main(){
	int K, N, F;
	cin >> K >> N >> F;
	vector<int> A(F);
	int sum = 0;
	rep(i, F){
		cin >> A[i];
		sum += A[i];
	}
	
	if(K*N-sum < 0){
		cout << -1 << endl;
	}
	else{
		cout << K*N-sum << endl;
	}
	
	
}