#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vl = vector<ll>;
using vvl = vector<vl>;
using pl = pair<ll, ll>;
const ll INF = ll(1e18);
const ll mod = ll(998244353);
const double pi = acos(-1);
#define rep0(i,n) for(ll (i) = 0; (i) < (n); ++(i))
#define rrep0(i,n) for(ll (i) = (n) - 1; (i) >= 0; --(i))
#define rep1(i,n) for(ll (i) = 1; (i) <= (n); ++(i))
#define rrep1(i,n) for(ll (i) = (n); (i) >= 1; --(i))
#define nfor(i,a,b) for(ll (i) = (a); (i) < (b); ++(i))
#define rnfor(i,a,b) for(ll (i) = (b) - 1; (i) >= (a); --(i))
#define pf(x) cout << (x) << endl
#define all(x) (x).begin(),(x).end()
#define yes pf("Yes")
#define no pf("No")
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
const int dx[8]={1,0,-1,0,1,1,-1,-1};
const int dy[8]={0,1,0,-1,1,-1,1,-1};

int multipf(vector<string>& s){
    cout << s[0];
    rep1(i, s.size() - 1)cout << " " << s[i];
    cout << endl;
    return 0;
}

int multipf(vector<ll>& n){
    cout << n[0];
    rep1(i, n.size() - 1)cout << " " << n[i];
    cout << endl;
    return 0;
}

ll gcd(ll a,ll b){
    if(a < b)swap(a, b);
    if(b == 0) return a;
    return gcd(b,a%b);
}
 
ll lcm(ll a,ll b){
    ll g = gcd(a,b);
    return a / g * b;
}

ll factorial(ll n){
    ll ans = 1;
    rep1(i, n){
        ans *= i;
        ans %= mod;
    }
    return ans;
}

ll power(ll a, ll b){
    ll ans = 1;
    while(b) {
        if(b & 1LL) ans = ans * a % mod;
        ans %= mod;
        a = a * a;
        a %= mod;
        b >>= 1;
    }
    return ans % mod;
}

//modの値の確認をすること
int main(){
    vl a(4);
    rep0(i, 4)cin >> a[i];
    sort(all(a));
    if(a[0] + a[1] + a[2] == a[3] || a[0] + a[3] == a[1] + a[2])yes;
    else no;
    return 0;
}

#include<bits/stdc++.h>

using namespace std;

#define boost ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define ll long long int
#define vi vector<int>
#define vvi vector<vi>
#define pii pair<int, int>
#define vii vector<pii>
#define pb push_back
#define B begin()
#define E end()
#define F first
#define S second
#define loop(z, v) for(auto &z: v)
#define nl "\n"
#define fat 1000000007

int main()
{
    boost
    int n, k, m;
    cin >> n >> k >> m;

    vi a(n - 1);
    loop(z, a) cin >> z;

    ll sum = accumulate(a.B, a.E, 0);

    int ans = m*n - sum;
    if(ans < 0) cout << 0 << nl;
    else if(ans <= k) cout << ans << nl;
    else cout << -1 << nl;
    return 0;
}