#include <iostream>
using namespace std;
int main() {
    int k, n, f, i, j;
    cin >> k >> n >> f;
    int b = k * n;
    int a[f];
    for (i = 0; i < f; i++) {
        cin >> a[i];
    }
    for (j = 0; j < f; j++) {
        b -= a[j];
    }
    if (b < 0) {b = -1;}
    cout << b << endl;
    return 0;
}