#include <iostream>

using namespace std;

int main() {
    int k, n, f;
    cin >> k >> n >> f;
    int sum = 0;
    for (int i = 0; i < f; ++i) {
        int age;
        cin >> age;
        sum += age;
    }
    int beanNum = k * n;
    int remain = beanNum - sum;
    if (remain < -1) {
        remain = -1;
    }
    cout << remain << endl;
}