// No.143 豆
// https://yukicoder.me/problems/no/143
//
#include <iostream>
using namespace std;

int main() {
    int K, N, F;
    cin >> K >> N >> F;
    
    int bean_total = K * N;
    for (auto i = 0; i < F; ++i) {
        int tmp;
        cin >> tmp;
        bean_total -= tmp;
    }
    
    cout << max(bean_total, -1) << endl;
}