#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define all(x) (x).begin(), (x).end()

int main() {
    int k, n, f;
    cin >> k >> n >> f;
    int beans = k * n;
    int total = 0;
    rep(i, f) {
        int a;
        cin >> a;
        total += a;
    }
    int res = beans - total;
    cout << (res >= 0 ? res : -1) << endl;
    return 0;
}