#include<bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(n);++i)
using namespace std;

int main(){
 int N, K, M;
 cin >> N >> K >> M;
 int tmp, sum = 0;
 int target = M * N;

 rep(i,N-1){
  cerr << "target " << target << endl;
  cin >> tmp;
  target -= tmp;
 }
 int ans;
 if(target <= 0){
  ans = 0;
 } else if(target <= K){
  ans = target;
 } else {
  ans = -1;
 }
 cout << ans << endl;

}
#include<iostream>
#include<algorithm>
using namespace std;
int main(){
	int N,K,M,A,sum=0;
	cin>>N>>K>>M;
	for(int i=0;i<N-1;i++){
		cin>>A;
		sum+=A;
	}
	if(N*M-sum<=K)cout<<max(N*M-sum,0);
	else cout<<-1;
	return 0;
}