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

using namespace std;

int main() {
    int32_t k, n, f;
    cin >> k >> n >> f;
    int32_t sum = 0;
    for (auto i = 0; i < f; ++i) {
        int32_t a;
        cin >> a;
        sum += a;
    }
    cout << max(k * n - sum, -1) << endl;
    return 0;
}