#include <bits/stdc++.h>
using namespace std;

int main(void){
    int k, n, f;
    cin >> k >> n >> f;
    vector<int> a(f);
    for(auto& i : a) cin >> i;
    int b = k*n;
    int eat_b = accumulate(a.begin(), a.end(), 0);
    if((b = b - eat_b) >= 0) cout << b << endl;
    else cout << -1 << endl;
    return 0;
}