#include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>

int main () {
    int K, N, F, age;
    int age_sum = 0;
    std::vector<int> A;

    std::cin >> K >> N >> F;

    for(int i = 0; i < F; i++) {
        std::cin >> age;
        A.emplace_back(age);
    }

    age_sum = std::accumulate(A.begin(), A.end(), 0);

    if (K * N - age_sum >= 0) {
        std::cout << K * N - age_sum << std::endl;
    }

    else {
        std::cout << -1 << std::endl;
    }

    return 0;
}