#include <iostream>

int main() {
    int K, N, F;
    int A[100] = { 0 };
    int sum_pea = 0;
    int sum_age = 0;
    
    std::cin >> K >> N >> F;
    
    for (int i=0; i<F; i++) {
        std::cin >> A[i];
        sum_age += A[i];
    }
    
    sum_pea = K * N;
    
    if (sum_pea < sum_age) std::cout << -1 << std::endl;
    else std::cout << (sum_pea - sum_age) << std::endl;

	return 0;
}