#include <bits/stdc++.h>
#define ll long long
#define ld long double
#define all(v) v.begin(), v.end()
#define sz size()
#define mp make_pair
#define pb push_back
#define rep(p, a, b) for (ll p = a; p < b; p++)
#define F first
#define S second
using namespace std;

typedef vector<ll> vll;
typedef vector<ld> vld;
typedef pair<ll, ll> pll;

int main()
{
    ll t, i, j, k, ans, n, m, sum = 0;

    //cin>>t;
    t = 1;
    while (t--)
    {
        cin >> n >> k >> m;
        rep(p, 0, n - 1)
        {
            cin >> i;
            sum += i;
        }

        ll mi = m * n - sum;
        if (mi > k)
            cout << "-1";
        else
            cout << max(0LL, mi);
    }

    return 0;
}

#include <bits/stdc++.h>
using namespace std;
using ll=long long;
 
int main() {
 
  int N,K,M; cin >> N >> K >> M;
  
  vector<int> v(N-1);
  int A = 0;
  for(int i = 0; i< N-1; i++){
    cin >> v.at(i);
    A += v.at(i);
  }
  
  if(K >= N * M - A){
    cout << max(0,N * M - A) << endl;
  }else{
    cout << "-1" << endl;
  }

}