#include <bits/stdc++.h>
using namespace std;

int main(){
    int n, k, m;

    int total=0,ans=0;
    cin >> n >> k >> m;
    int ar[n];
    int sum = 0;
    for(int i = 0; i < n - 1; i++)
    {
        cin >> ar[i];
        sum = sum+ar[i];
    }
    total = (n * m - sum);

    if (total <= 0) ans = 0;
    else if(total <= k) ans = total;
    else ans = -1;
    cout << ans << endl;
}

#include <bits/stdc++.h>

using namespace std;

#define fastio ios::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL)
#define t_times int t; cin >> t; while(t--)
#define _parr(a, n) for(int __i = 0; __i < (int )n; __i++) cout << a[__i] << ' '; cout << '\n'
#define fr(i, p, n) for(int i = (int )p; i < (int )n; i++)
#define frn(i, p, n) for(int i = (int )p; i <= (int )n; i++)
#define rev(i, n, p) for(int i = (int )n; i >= (int )p; i--)
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).begin(), (c).end()
#define sci(n) scanf("%d", &n)
#define scl(n) scanf("%lld", &n)
#define prin(n) printf("%d\n", n)
#define prln(n) printf("%lld\n", n)
#define mp make_pair
#define F first
#define S second
#define pb push_back

typedef long long int LL;
typedef unsigned long long ULL;
typedef unsigned int uint;
typedef pair< int, int> ii;
typedef vector< int> vi;

const int mod = 1e9 + 7;
const LL INF = 9e18 + 2e17;
const int inf = 2e9;
const int N = 2e5 + 22;
const double eps = 1e-10;

int n, m, k;

void _solve() {
	cin >> n >> k >> m;
	int x, sum = 0;
	fr(i, 1, n) cin >> x, sum += x;
	int z = m * n;
	if(z > sum && (z - sum) > k) cout << -1;
	else if(z > sum) cout << z - sum;
	else cout << 0;
}

int main() {
    fastio;
    //t_times
    _solve();
    return 0;
}