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

int main() {
    int n ,k, f;
    cin >> n >> k >> f;
    int sum = 0;
    for (int i = 0;i < f; i++) {
        int a;
        cin >> a;
        sum += a;
    }
    int ans = n * k - sum;
    if (ans >= 0) {
        cout << ans << endl;
    } else {
        cout << -1 << endl;
    }
    
    return 0;
}