#include <iostream>
#include <vector>

int main() {
    int k, n, f;
    int tmp;
    
    std::cin >> k >> n >> f;
    int b = k * n;

    for (int i = 0; i < f; i++) {
        std::cin >> tmp;
        b -= tmp;
    }
    if (b < 0) {
        std::cout << -1 << std::endl;
    } else {
        std::cout << b << std::endl;
    }

}