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 0;
    }
}