#include <bits/stdc++.h>
const int inf = int(1e9);
const int mod = inf + 7;
const double PI = acos(-1.0);
using namespace std;
const int N = 1e5 + 155;
void solve() {
  int n, k, s = 0;
  cin >> n >> k;
  for (int i = 0; i < n; ++i) {
    int x;
    cin >> x;
    s += x;
  }
  for (int ans = 0;; ans++) {
    int a = 2 * (s + ans * k);
    int b = (2 * k - 1) * (ans + n);
    if (a >= b) {
      cout << ans;
      return;
    }
  }
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t = 1;
  while (t--) solve();
  return 0;
}
