#include <bits/stdc++.h>
using namespace std;
#define REP(i,n) for (int i = 0; i < n; i++)
typedef long long ll;
// Welcome to my source code!

int main() {
    int k, n, f;
    cin >> k >> n >> f;
    int A[f], sum = 0;
    REP(i,f) {
        cin >> A[i];
        sum += A[i];
    }
    cout << (k*n >= sum ? k*n-sum : -1) << endl;
}