#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define endl "\n"

int main() {
    int k, n, f;
    cin >> k >> n >> f;
    int sum = k * n;
    for (int i = 0; i < f; ++i) {
        int a;
        cin >> a;
        sum -= a;
    }

    cout << ((sum >= 0) ? sum : -1) << endl;
    return 0;
}