#include <iostream>
#include <vector>

using namespace std;
using ll=long long;

int main(){
    ll n,totsum;
    cin>>n>>totsum;
    vector<ll> s(n);
    ll csum=0;
    for(int i=0;i<n;i++){
        cin>>s[i];
        csum+=s[i];
    }
    for(int i=0;i<n;i++){
        if(s[i]&1) continue;
        ll cur=csum-s[i]/2;
        if(cur==totsum*(n-1)){
            cout<<s[i]/2<<endl;
            return 0;
        }
    }

    return 0;
}