#define PROBLEM no143
// "Env.h"ヘッダが存在していたらインクルードする
#if __has_include("../Env.h")
#include "../Env.h"
#endif

#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <unordered_set>
#include <unordered_map>
#include <algorithm>
#include <cmath>

using namespace std;
using VI = vector<int>;
using VVI = vector<VI>;
using USI = unordered_set<int>;

class no143 {
public:
    void solve() {
        // std::cout << "Hello, no143" << std::endl;

        int ans;
        int K, N, F;
        cin >> K;
        cin >> N;
        cin >> F;

        int total_mame = K*N;

        int a;
        int sum_a = 0;
        for (int i = 0; i < F; ++i) {
            cin >> a;
            sum_a += a;
        }
        ans = total_mame >= sum_a ? total_mame - sum_a : -1;

        cout << ans << endl;

    }
};


#ifndef YUKICODER_ENV_H

int main() {
    auto problem = new PROBLEM();
    problem->solve();
    return 0;
}

#endif //YUKICODER_ENV_H