#include<bits/stdc++.h>
using namespace std;

#define FOR(i, a, b) for(int i = a; i < b; ++i)
#define REP(i, n) FOR(i, 0, n)
#define _ << " " <<
#define sz(x) ((int) x.size())
#define pb(x) push_back(x)
#define TRACE(x) cerr << #x << " = " << x << endl

typedef long long ll;
typedef pair<int, int> point;

const int mod = 1e9 + 7;

int add(int x, int y) {x += y; if(x >= mod) return x - mod; return x;}
int sub(int x, int y) {x -= y; if(x < 0) return x + mod; return x;}
int mul(int x, int y) {return (ll) x * y % mod;}

const int MAXN = 105;

int n, k, m;

int main(){
  ios_base::sync_with_stdio(false); cin.tie(0);

  cin >> n >> k >> m;
  int sum = 0;
  REP(i, n - 1){
    int x; cin >> x;
    sum += x;
  }
  if(sum / n >= m)
    cout << 0;
  else if((sum + k) / n < m)
    cout << -1;
  else
    cout << m * n - sum;
}

#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#include <bits/stdc++.h>
#define FAST_IO  ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define mod 1000000000
#define endl "\n"
#define PI 3.14159265358979323846264338327950288

using namespace std ;
using bigint = long long int ;
using vi = vector<int> ;
using vll = vector<long long int> ;


int main(){
    FAST_IO ;
    int N , K , M ; cin >> N >> K >> M ;
    int sum = 0 ;
    for( int i = 0 ; i < N - 1 ; i++ ){
        int x ; cin >> x ;
        sum += x ;
    }
    int ans = N * M - sum ;
//    cout << ans << endl ;
    if( ans > K ){
        cout << "-1" << endl ;
        return 0 ;
    }
    cout << max( 0 ,ans ) << endl ;
    return 0 ;
  }

