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

const int INF = INT32_MAX;

const ll MOD = 1e9+7;

signed main()
{
    int K, N, F;
    cin >> K >> N >>F;
    ll a = 0;
    rep(i,F){
        ll b;
        cin >> b;
        a+= b;
    }
    a = K*N - a;
    if(a < 0)a = -1;
    cout << a << endl;
    return 0;

}