#include <bits/stdc++.h>

using namespace std;

#define REP(i, n) for(int i = 0; i < n; i++)

int main() {
    int k, n, f;
    cin >> k >> n >> f;
    int a[f];
    REP(i, f)cin >> a[i];
    int x = k * n;
    REP(i, f) x -= a[i];

    if (x < 0) {
        cout << -1 << endl;
    } else {
        cout << x << endl;
    }
    return 0;
}