//@formatter:off
#include<bits/stdc++.h>
#define overload4(_1,_2,_3,_4,name,...) name
#define rep1(i,n) for (ll i = 0; i < ll(n); ++i)
#define rep2(i,s,n) for (ll i = ll(s); i < ll(n); ++i)
#define rep3(i,s,n,d) for(ll i = ll(s); i < ll(n); i+=d)
#define rep(...) overload4(__VA_ARGS__,rep3,rep2,rep1)(__VA_ARGS__)
#define rrep(i,n) for (ll i = ll(n)-1; i >= 0; i--)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define popcount(x) __builtin_popcount(x)
#define pb push_back
#define eb emplace_back
#ifdef __LOCAL
#define debug(...) { cout << #__VA_ARGS__; cout << ": "; print(__VA_ARGS__); cout << flush; }
#else
#define debug(...) void(0)
#endif
#define INT(...) int __VA_ARGS__;scan(__VA_ARGS__)
#define LL(...) ll __VA_ARGS__;scan(__VA_ARGS__)
#define STR(...) string __VA_ARGS__;scan(__VA_ARGS__)
#define CHR(...) char __VA_ARGS__;scan(__VA_ARGS__)
#define DBL(...) double __VA_ARGS__;scan(__VA_ARGS__)
#define LD(...) ld __VA_ARGS__;scan(__VA_ARGS__)
using namespace std;
using ll = long long;
using ld = long double;
using P = pair<int,int>;
using LP = pair<ll,ll>;
using vi = vector<int>;
using vvi = vector<vector<int>>;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using vd = vector<double>;
using vvd = vector<vector<double>>;
using vs = vector<string>;
using vc = vector<char>;
using vvc = vector<vector<char>>;
using vb = vector<bool>;
using vvb = vector<vector<bool>>;
using vp = vector<P>;
using vvp = vector<vector<P>>;
template<class S,class T> istream& operator>>(istream &is,pair<S,T> &p) { return is >> p.first >> p.second; }
template<class S,class T> ostream& operator<<(ostream &os,const pair<S,T> &p) { return os<<'{'<<p.first<<","<<p.second<<'}'; }
template<class T> istream& operator>>(istream &is,vector<T> &v) { for(T &t:v){is>>t;} return is; }
template<class T> ostream& operator<<(ostream &os,const vector<T> &v) { os<<'[';rep(i,v.size())os<<v[i]<<(i==int(v.size()-1)?"":","); return os<<']'; }
template<class T> void vecout(const vector<T> &v,char div='\n') { rep(i,v.size()) cout<<v[i]<<(i==int(v.size()-1)?'\n':div);}
template<class T> bool chmin(T& a,T b) {if(a > b){a = b; return true;} return false;}
template<class T> bool chmax(T& a,T b) {if(a < b){a = b; return true;} return false;}
void scan(){}
template <class Head, class... Tail> void scan(Head& head, Tail&... tail){ cin >> head; scan(tail...); }
template<class T> void print(const T& t){ cout << t << '\n'; }
template <class Head, class... Tail> void print(const Head& head, const Tail&... tail){ cout<<head<<' '; print(tail...); }
template<class... T> void fin(const T&... a) { print(a...); exit(0); }
const string yes[] = {"no","yes"};
const string Yes[] = {"No","Yes"};
const string YES[] = {"NO","YES"};
const int inf = 1001001001;
const ll linf = 1001001001001001001;
//@formatter:on

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    INT(n, x);
    vi s(n);
    cin >> s;
    ll sum = accumulate(all(s), 0LL);
    rep(i, n) {
        if (s[i] & 1) continue;
        ll tmp = sum - s[i] / 2;
        if (tmp % (n - 1)) continue;
        if (tmp / (n - 1) == x) {
            fin(s[i] / 2);
        }
    }
}