#include <iostream>
#include <math.h>
#include <vector>
#include <string>
#include <numeric>
#include <unordered_set>
#include <iomanip>
#include <algorithm>
#include <functional>
#include <map>
using namespace std;

int main(){
    int n,k,m; cin >> n >> k >> m;
    vector<int> a(n-1);
    for(int i=0;i<n-1;i++) cin >> a[i];
    int sum=0;
    for(int i=0;i<n-1;i++) sum += a[i];
    int goal = m * n;
    if( goal > sum + k ) cout << -1 << endl;
    else cout << max(0,goal - sum) << endl;

}
#include<bits/stdc++.h>
using namespace std;
using ll = unsigned long long;

int main(){
    int n,k,m;
    while( cin >> n >> k >> m ){
        int arr[n];
        int sum = 0;
        for( int i = 0; i < n-1; i++ ){
            int a;
            cin >> a;
            sum += a;
        }
        bool flag = false;
        for( int i = 0; i <= k; i++ ){
            int temp = sum+i;
            if( temp/n >= m ){
                cout << i << endl;
                flag = true;
                break;
            }
        }
        if( flag == false ){
            cout << "-1\n";
        }
    }
}
