#include <bits/stdc++.h>

using namespace std;

int main(){
	int n, l, i, pos;

	cin >> n >> l;
	std::vector<int> a(l+1, 0);

	for(i=0; i<n; i++){
		cin >> pos;
		a[pos] = 1;
	}

	double r_max = 0;
	int j;

	for(i=0; i<=l; i++){
		if(a[i] == 1){
			double r = 0;
			for(j=1; j<=i; j++){
				if(a[i-j] != 1) r++;
				else r = r/2;
				if(r > r_max) r_max = r;
			}
		}
	}

	cout << r_max << endl;

	return 0;
}