#include <iostream>
using namespace std;

int main() {
    int k, n, f;
    cin >> k >> n >> f;
    int has_beans = k * n;

    int needed_beans = 0;
    for (int i = 0; i < f; ++i) {
        int age; cin >> age;
        needed_beans += age;
    }

    if (has_beans < needed_beans) {
        cout << -1 << endl;
    } else {
        cout << has_beans - needed_beans << endl;
    }
}