/**
*    author:  boutarou
*    created: 02.08.2020 11:34:55
**/

#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < int(n); i++)
using ll = long long;
using P = pair<int, int>;

int main() {
    int k, n, f;
    cin >> k >> n >> f;
    int exist = k * n;
    int need = 0;
    rep(i, f) {
        int x;
        cin >> x;
        need += x;
    }
    cout << max(-1, exist - need) << endl;
    return 0;
}