#include<iostream>
using namespace std;

int main()
{
    int k,n,f;
    cin >> k >> n >> f;
    int sum = k*n;
    int age, sum_age=0;
    for(int i=0; i<f; i++) {
        cin >> age;
        sum_age += age;
    }
    cout << (sum-sum_age>=0 ? sum-sum_age : -1) << endl;
    return 0;
}