#include<bits/stdc++.h>
#define rep(i, n) for (int i=0;i<(n);i++)
using namespace std;
#define vecout(V) for(int i=0;i<V.size();i++) {cout<<V[i]<<endl;}
typedef long long ll;

int main(void){
    int n, k, m;
    cin>>n>>k>>m;
    vector<int> v(n-1);
    rep(i, n-1) cin>>v[i];
    int sum=accumulate(v.begin(), v.end(), 0);
    int ans=n*m-sum;
    if(ans<=k) cout<<max(0, ans)<<endl;
    else cout<<-1<<endl;
    return 0;
}
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
	int N, K, M;
	cin >> N >> K >> M;
	vector<int> A(N);
	int ans = 0, sum = 0;
	for (int i = 0; i < N - 1; i++)
	{
		cin >> A[i];
		sum += A[i];
	}
	ans = M * N - sum;
	if (ans > K) {
		cout << "-1" << endl;
	}
	else if (ans <= 0) {
		cout << "0" << endl;
	}
	else {
		cout << ans << endl;
	}
}