#include <bits/stdc++.h>
using namespace std;
#define rep(i, n, m) for (ll i = n; i < (ll)(m); i++)
#define rrep(i, n, m) for (ll i = n; i <= (ll)(m); i++)
using ll = long long;

int main() {
    // 豆
    // K * N
    // F A1,A2,...,AF
    //
    ll k, n, f;
    cin >> k >> n >> f;
    ll sum = 0;
    ll tmp;
    rep(i, 0, f) {
        cin >> tmp;
        sum += tmp;
    }

    if (sum > k * n)
        cout << -1 << endl;
    else
        cout << k * n - sum << endl;
}