#include <bits/stdc++.h>
#define rep(i, n) for (int i = 1; i <= (n); i++)
using namespace std;
typedef long long ll;

int main(){
    int k, n, f;
    cin >> k >> n >> f;
    vector<int> a(n);
    rep(i ,f) cin >> a[i];
    int total = k * n;
    rep(i, f){
        total -= a[i];
        if(total < 0){
            cout << "-1" << endl;
            return 0;
        }
    }
    cout << total << endl;
    return 0;
}