#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <stack>
#include <algorithm>
#include <string>
#include <map>
#include <iterator>
#include <set>
#include <queue>

using namespace std;


int main() {


    int K, N, F;
    cin >> K >> N >> F;
    int s = K * N;
    for (int i = 0; i < F; ++i) {
        int A;
        cin >> A;
        s -= A;
    }

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

    return 0;
}