# include <iostream>
# include <iomanip>
# include <algorithm>
#include <regex>

using namespace std;


int main() {
    int k, n, f;
    cin >> k >> n >> f;
    int age[f];
    int mame = k * n;
    for (int i = 0; i < f; i++) cin >> age[i];

    for (int i = 0; i < f; i++) {
        mame -= age[i];
        if (mame < 0) {
            mame = -1;
            break;
        }
    }

    cout << mame << endl;

}