#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
using Graph = vector<vector<int>>;
using ll = long long;
//正解に近いコードはこちらに移しておく

int main() {
    int k,n,f;
    cin >> k >> n >> f;
    //それぞれの年齢
    vector<int> A(f);
    rep(i,f) cin >> A[i];
    //年齢の合計
    int age = accumulate(A.begin(),A.end(),0);
    if (age > k*n) {
        cout << -1 << endl;
    }
    else {
        cout << k*n - age << endl;
    }
}