// No.143 豆
#include <iostream>
using namespace std;

int main() {
    int N, K, F;
    cin >> N >> K >> F;
    int res = N * K;
    for (int i = 0; i < F; ++i) {
        int A;
        cin >> A;
        res -= A;
    }
    if (res < 0) cout << -1 << endl;
    else cout << res << endl;
}