#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep1(i, n) for (int i = 1; i < (int)(n + 1); i++)
using ll = long long;
using P = pair<int, int>;
using Graph = vector<vector<int>>;

int main()
{
    int k, n, f; cin >> k >> n >> f;
    int beans = k*n;
    rep(i, f){
        int a; cin >> a;
        beans -= a;
    }

    if(beans >= 0){
        cout << beans << endl;
    }
    else{
        cout << -1 << endl;
    }

}