/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author aryssoncf
 */



// Taken from: https://github.com/bqi343/USACO/blob/master/Implementations/content/contest/templateShort.cpp

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pi;
typedef vector<int> vi;
typedef vector<ull> vul;
typedef vector<ll> vl;
typedef vector<pi> vpi;

#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define F0R(i, a) FOR(i,0,a)
#define ROF(i, a, b) for (int i = (b)-1; i >= (a); --i)
#define R0F(i, a) ROF(i,0,a)
#define trav(a, x) for (auto& a: x)

#define sz(x) (int)x.size()
#define all(x) begin(x), end(x)
#define rsz resize

#define mp make_pair
#define pb push_back
#define f first
#define s second

const int MOD = 1e9 + 7; // 998244353; // = (119<<23)+1
const int MX = 2e5 + 5;

template<class T>
bool ckmin(T &a, const T &b) {
    return a > b ? a = b, 1 : 0;
}

template<class T>
bool ckmax(T &a, const T &b) {
    return a < b ? a = b, 1 : 0;
}

mt19937 rng((uint32_t) chrono::steady_clock::now().time_since_epoch().count());


//
// Created by aryssoncf on 28/10/2019.
//

#include <type_traits>


#include <iterator>
#include <utility>

template<typename T>
constexpr auto hasBegin(int) -> decltype(std::begin(std::declval<T>()), true) {
    return true;
}

constexpr bool hasBegin(...) {
    return false;
}

template<typename T>
using IsContainer = std::integral_constant<bool, hasBegin<T>(0)>;


#ifndef JHELPER_EXAMPLE_PROJECT_IO_HPP
#define JHELPER_EXAMPLE_PROJECT_IO_HPP

#endif //JHELPER_EXAMPLE_PROJECT_IO_HPP

template<typename T>
std::istream &operator>>(std::istream &is, std::vector<T> &vec) {
    for (T &element: vec) {
        is >> element;
    }
    return is;
}

template<typename T, typename U>
std::istream &operator>>(std::istream &is, std::pair<T, U> &p) {
    is >> p.first;
    is >> p.second;
    return is;
}

template<class X, class Y>
std::ostream &operator<<(std::ostream &os, std::pair<X, Y> const &p) {
    return os << p.first << " " << p.second;
}

template<class Ch, class Tr, class Container>
std::basic_ostream<Ch, Tr> &operator<<(std::basic_ostream<Ch, Tr> &os, Container const &x) {
    bool first = true;
    for (auto &y : x) {
        if (first) {
            first = false;
        } else {
            os << " ";
        }
        os << y;
    }
    return os;
}

using namespace std;

/*
PROG:
LANG: C++
*/

class BAchieveTheGoal {
public:
    void solve(std::istream &in, std::ostream &out) {
        int n, k, m;
        in >> n >> k >> m;
        int tot = n * m;
        vi A(n - 1);
        in >> A;
        int sum = accumulate(all(A), 0), res = max(tot - sum, 0);
        if (res > k) {
            res = -1;
        }
        out << res << "\n";
    }

    void setup() {}
};


int main() {
    std::ios_base::sync_with_stdio(false);
    BAchieveTheGoal solver;
    solver.setup();
    std::istream &in(std::cin);
    std::ostream &out(std::cout);
    in.tie(nullptr);
    out << std::fixed;
    out.precision(20);
    solver.solve(in, out);
    return 0;
}

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<iostream>
#include<string>
#include<vector>
#include<stack>
#include<bitset>
#include<cstdlib>
#include<cmath>
#include<set>
#include<list>
#include<deque>
#include<map>
#include<queue>
#define INF 0x3f3f3f3f
using namespace std;
typedef long long ll;
int main(void)
{
    int T,t,x,y,n,m,k,s=0;
    int a[2000]={0};
    scanf("%d %d %d",&n,&k,&m);
    for(int i=0;i<n-1;i++)
    {
        scanf("%d",&a[i]);
        s+=a[i];
    }
    if(m*n-s<=k)
    {
        for(int i=0;i<=k;i++)
        {
            if(s+i>=m*n)
            {
                printf("%d\n",i);
                break;
            }
        }
    }
    else
        printf("%d\n",-1);
    return 0;
}







