#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <iomanip>
#include <random>
#include <cstdio>
#include <cmath>
#include <map>
using namespace std;
typedef long long ll;
#define rep(i, n) for(int i = 0; i < (int) n; i++)

int main() {
    int n,k,m;
    cin >> n >> k >> m;
    int sum = 0;
    rep(i,n-1) {
        int a;
        cin >> a;
        sum += a;
    }
    for (int i = 0; i <= k; i++)
    {
        if ((sum+i)/n >= m) {
            cout << i << endl;
            return 0;
        } 
    }
    cout << -1 << endl;
    return 0;
}
#include <iostream>

using namespace std;

int main(){
  int n,k,m;
  cin >> n >> k >> m;
  int Score[n-1],sum=0;
  for(int i=0;i<n-1;i++){
    cin >> Score[i];
    sum += Score[i];
  }
  //主処理
  int  TargetScore = m*n; 
  //すでに目標点を超えている場合
  if(TargetScore <= sum){
    cout << 0 << endl;
  }
  //下回っている場合
  else{
    if(TargetScore-sum <= k){
       cout << TargetScore-sum << endl;
    }
    else{
       cout << -1 << endl;
    }
  }
  
  return 0;
}