#include <bits/stdc++.h>
using namespace std;
int main() {
  double s, o = 0, f = 0;
  double x, y;
  cin >> x >> y;
  for (int i = 0; i < x; i++) {
    cin >> s;
    o = o + s;
  }
  double n = y - .5;
  while ((o / x) < n) {
    o = o + y;
    x++;
    f++;
  }
  cout << f << endl;
  return 0;
}
