#include "bits/stdc++.h"
#define rep(i,n) for(int i=0; i<(n); ++i)
#define fo(i,a,b) for(int i=a; i<(b); ++i)
#define out(a) cout << a << endl
#define so(v) sort(v.begin(),v.end())
#define revso(v) sort(v.begin(),v.end(), greater<int>())
using namespace std;
using ll = long long;

int main() 
{
	int n, k, m; cin >> n >> k >> m;
	
	int res = n * m;
	rep(i, n-1)
	{
		int a; cin >> a;
		res -= a;
	}
	res = max(0, res);
	if (res > k) res = -1;
	out(res);

}
#include<bits/stdc++.h>
using namespace std;
int n;
int m;
int k;

int main(){
    cin >> n >> k >> m;
    int top = 0;
    for(int i = 0;i<n-1;i++){
        int s;
        cin >> s;
        top+=s;
    }
    int gereken = n*m;
    if(top > gereken){
        cout << "0";
        return 0;
    }
    int suanki = gereken-top;
    if(suanki > k){
        cout << "-1";
        return 0;
    }
    cout << suanki;

}
