#include <iostream>
#include <string>

using namespace std;

static void setup()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
}

static void run()
{
    int k, n, f;
    cin >> k >> n >> f;

    auto sum = 0;
    for (auto i = 0; i < f; ++i) {
        int a;
        cin >> a;
        sum += a;
    }

    const auto r = k * n - sum;
    cout << (r >= 0 ? r : -1) << endl;
}

int main(int argc, char **argv)
{
    setup();
    run();
    return 0;
}