#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef string str;
int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    ll n, k, f;
    cin >> n >> k >> f;
    ll sum = 0;
    for (ll i = 0; i < n; i++)
    {
        ll x;
        cin >> x;
        sum += x;
    }
    if (sum > n * k) cout << -1;
    else cout << n * k - sum;
    return 0;
}