#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <map>
#include <cmath>
#include <iomanip>
#include <bitset>
using namespace std;
#define REP(i,n) for (int i=0;i<(n);++i)
#define rep(i,a,b) for(int i=a;i<(b);++i)
template<class T> inline bool chmin(T &a, T b){ if(a > b) { a = b; return true;} return false;}
template<class T> inline bool chmax(T &a, T b){ if(a < b) { a = b; return true;} return false;}
using ll = long long;
constexpr ll INF = 1LL << 60;
constexpr int MOD = 998244353;


int main() {
  ios::sync_with_stdio(false); cin.tie(0);
  ll N, K, M; cin >> N >> K >> M;
  vector<ll> A(N);
  for(int i = 0; i < N-1; ++i) cin >> A[i];

  ll f = N * M;
  REP(i, N-1) f -= A[i];
  
  if (f > K) {
    cout << -1;
  } else {
    cout << max(0LL, f);
  }
  
  return 0;
}

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

int main()
{
  int n,k,m,s=0,i;

  cin>>n>>k>>m;
    int arr[n];


  for(i=0;i<n-1;i++)
  {
    cin>>arr[i];
    s+=arr[i];
  }
  int ans=n*m-s;
  if(ans>k)
  cout<<"-1"<<endl;
  else if(ans<0)
  cout<<"0"<<endl;
  else
  cout<<ans<<endl;

}
