#include <bits/stdc++.h>
using namespace std;
#define rep(i, s, n) for (int i = (s); i < (n); ++i)
#define rem(i, s, n) for (int i = (s); i >= (n); --i)
using ll = long long;
const int MOD = 1e9 + 7;

int main(){
    int n, k, m; cin >> n >> k >> m;
    vector<int> a(n); rep(i, 0, n) cin >> a[i];

    int sum = accumulate(a.begin(), a.end(), 0);
    int g = n * m;
    if (g - sum > k) cout << -1 << endl;
    else cout << max((g - sum), 0) << endl;

    return 0;
}
#include<iostream>

using namespace std;

int main(){
    int n, k, m;
    cin >> n >> k >> m;
    int sum = 0;
    for (int i = 0; i < n-1; i++)
    {
        int ai;
        cin >> ai;
        sum += ai;
    }
    int ans = n * m - sum;
    if (ans <= k)
    {
        if(ans<0)
            ans = 0;
        cout << ans << endl;
        return 0;
    }
    cout << "-1" << endl;
    return 0;
}