#include <iostream>
#include <math.h>
#include <algorithm>
#include <functional>
#include <vector>
#include <typeinfo>
#include <climits>
using namespace std;
#define INIT cin.tie(0); ios::sync_with_stdio(false);
#define FOR(i, a, n) for (int i = a; i < n; i++)
#define REP(i, n) for(int i = 0; i < n; i++)

int main()
{
    INIT;

    int k, n, f;
    cin >> k >> n >> f;
    int A[f];
    REP(i, f) cin >> A[i];

    int total = k * n;
    int flag = 0;
    REP(i, f){
        total -= A[i];
        if(total < 0){
            flag = 1;
            break;
        }
    }
    if(flag == 1){
        cout << -1 << endl;
    }else cout << total << endl;



    return 0;
}