//#include "pch.h"
#include <stdio.h>
#include <string.h>
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <functional>
#include <map>
#include <iomanip>
#include <math.h> 
#include <stack>
#include <queue>
#include <bitset>
#include <cstdlib>
#include <tuple>
#include <cctype>
#include <ctype.h>
#include <set>
#include <sstream>
#include <time.h>
#include <numeric>
using namespace std;
//#define int long long
#define rep(i,s,n) for(int i = s;i<n;i++)
#define repe(i,s,n) for(int i = s;i<=n;i++)
#define rrep(i,s,n) for(int i = (n)-1;i>=(s);i--)
#define all(v) (v).begin(),(v).end()
#define pb push_back
#define fi first
#define se second
#define chmin(a,b) a=min((a),(b))
#define chmax(a,b) a=max((a),(b))
#define l1 list[index]
#define l2 list[index - 1]
#define l3 list[index + 1]
#define iif(i,j) ((i<0 && j<0) || (i>0 && j>0)) ? true : false 
typedef long long ll;
typedef pair<int, int>pint;
typedef vector<int>vint;
typedef vector<pint>vpint;
typedef pair<pint, int> P1;
typedef pair<int, pint> P2;
typedef pair<pint, pint> PP;
static const ll maxLL = (ll)1 << 62;
const ll MOD = 1000000007;
const ll INF = 1e18;
const double PI = 3.14159265359;
int ca[12] = { 31,28,31,30,31,30,31,31,30,31,30,31 };




signed main() {
	int n, k, m;

	cin >> n >> k >> m;
	int sum = 0;

	for (int i = 0; i < n - 1; i++) {
		int num;
		cin >> num;
		sum += num;

	}


	for (int i = 0; i <= k; i++) {
		if ((sum + i)/n >= m) {
			cout << i << endl;
			return 0;
		}
	}

	cout << -1 << endl;

	return 0;
}
#include <bits/stdc++.h>
#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <stdio.h>
#include <cstdlib>
#include <math.h>
 
using namespace std;
typedef long long ll;
 
int main(){
  int n, k, m, sum=0, ans=0;
  cin >> n >> k >> m;
  vector<int>a(n);
  for(int i=0; i<n-1; i++){
  	cin >> a.at(i);
    sum += a.at(i);
  }
  ans = n*m-sum;
  if(ans <= 0){
  	cout << "0";
  }
  else if(ans <= k){
  	cout << ans;
  }
  else{
  	cout << "-1";
  }
}
