#include <iostream>
#include <cstdio>
#include <string>
#include <algorithm>
#include <utility>
#include <cmath>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <tuple>
#include <numeric>
#include <functional>
using namespace std;
typedef long long ll;
typedef vector<ll> vl;
typedef vector<vector<ll>> vvl;
typedef pair<ll, ll> P;
#define rep(i, n) for(ll i = 0; i < n; i++)
#define exrep(i, a, b) for(ll i = a; i <= b; i++)
#define out(x) cout << x << endl
#define exout(x) printf("%.10f\n", x)
#define chmax(x, y) x = max(x, y)
#define chmin(x, y) x = min(x, y)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define pb push_back
#define re0 return 0
const ll mod = 1000000007;
const ll INF = 1e16;
const ll MAX_N = 100010;

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

    ll sum = 0;
    rep(i, n-1) {
        ll a;
        cin >> a;
        sum += a;
    }

    ll x = m*n - sum;

    if(x > k) {
        out(-1);
    }
    else {
        out(max(0LL, x));
    }
    
    re0;
}
#include<stdio.h>
static inline int MPow(int a,int b,const int MOD){return b?1l*MPow(1l*a*a%MOD,b>>1,MOD)*(b&1?a:1)%MOD:1;}
int main(void){int A,B,C;if(!scanf("%d %d %d",&A,&B,&C)){return 1;}return !putchar(MPow(A,MPow(B,C,4)+4,10)+48);}