#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; ++i)
using ll = long long;
using namespace std;
const int INF = 1e9;

int main() {
    int K, N, F;
    cin >> K >> N >> F;

    int sum = K * N;
    rep(i,F){
        int a;
        cin >> a;
        sum -= a;
    }

    if(sum<0)
        cout << -1 << endl;
    else
        cout << sum << endl;

    return 0;
}