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

#define fi first
#define se second
#define pb push_back
#define rep(i,n) for(int i=0; i<(int)(n); i++)

int main() {

	int n,k,m;
	cin >> n >> k >> m;
	int a=0;
	int temp;
	rep(i,n-1) {
		cin >> temp;
		a += temp;
	}
	//cout << a << endl;
	rep(i,k+1) {
		if (((a + i)/n)>=m) {
			cout << i << endl;
			return 0;
		}
	}
	cout << "-1" << endl;
  return 0;
}

#include <iostream>
using namespace std;

int main(){
  int n, k, m, sum = 0;
  cin >> n >> k >> m;
  int *a = new int[n];
  for(int i=0;i<n-1;i++){
    cin >> a[i];
  }
  for(int i=0;i<n-1;i++){
    sum += a[i];
  }
  if(n*m-sum<=k){
    if(n*m-sum>0){
      cout << n*m-sum << endl;
    }else {
      cout << '0' << endl;
    }
  }else{
    cout << "-1" << endl;
  }
  return 0;
}