#include <bits/stdc++.h>
using namespace std;
typedef unsigned long long ll;
typedef pair<int, int> pii;

int main() {
  int N, K, M;
  cin >> N >> K >> M;
  M *= N;
  int sum = 0;
  for (int i = 0; i < N - 1; i++) {
    int a;
    cin >> a;
    sum += a;
  }
  if (M - sum > K) {
    cout << -1 << endl;
  } else {
    cout << max(0, M - sum) << endl;
  }
  return 0;
}
#include <iostream>
using namespace std;
int main(void){
    int n,k,m,ans,sum;
    ans = -1;
    sum = 0;
    cin >> n;
    cin >> k >> m;
    for(int i=1;i<n;i++){
       int temp;
       cin >> temp;
       sum += temp;
    }
    for(int i=0;i<= k;i++){
        if(m <= (sum + i)/n){
            ans = i;
            break;
        }
    }
    cout << ans << endl;
    return 0;
}

