#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int (i)=(0);(i)<(int)(n);++(i))
using ll = long long;
using namespace std;

int main() {
    int N, K, M;
    cin >> N >> K >> M;
    vector<int> A(N-1);
    int sum = 0;
    rep(i, N-1) {
        cin >> A[i];
        sum += A[i];
    }

    for (int i=0; i<=K; ++i) {
        double s = sum + i;
        if (s/N >= M) {
            cout << i << endl;
            return 0;
        }
    }

    cout << -1 << endl;


}

#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef pair<int,int> P;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    ll N,K,M; cin >> N >> K >> M;

    ll sum = 0;
    for (int i = 0;i < N-1;i++){
        ll a; cin >> a;
        sum += a;
    }

    ll goal = M*N;

    if(goal <= sum) cout << 0 << endl;
    else if(goal-sum > K) cout << -1 << endl;
    else cout << goal-sum << endl;

    return 0;

}