#include <bits/stdc++.h>
using namespace std;

int main() {
    int a,b,c;
    cin >> a >> b >> c;
    vector<int> d(c);
    int s = 0;
    for (int i=0; i<c; i++) {
        cin >> d[i];
        s = s + d[i];
    }

    int ss = a * b;
    int ans = ss - s;

    if (ans < 0) {
        cout << "-1" << endl;
        return 0;
    }
    cout << ans << endl;
    return 0;
}