#include <stdio.h>
#include <iostream>
#include <vector>

using namespace std;

#define LL long long

int main() {
  int N; cin >> N;
  LL X, sum = 0; cin >> X;
  
  vector<LL> S(N);

  for (int i = 0; i < N; i++) {
    cin >> S[i];

    sum += S[i];
  }

  for (int i = 0; i < N; i++) {
    if ((S[i] & 1) == 1) {
      continue;
    }

    if (2 * sum - S[i] == 2 * (N - 1) * X) {
      cout << S[i] / 2 << endl;

      break;
    }
  }

  return 0;
}