#include <iostream>
#include <algorithm>
#include <vector>
#include <cmath>
#include <string>
#include <iomanip>
#include <set>
#include <utility>
#include <vector>
#include <fstream>
#include <map>
#include <deque>
#include <queue>
#include <cstring>
#include <cstdio>
#include <list>
#include <bitset>

using namespace std;

const long long INF = 1e18;
const int MAXN = 1e5 + 1, MODM = 1e9 + 7;

#define ABS(x) ((x) < 0 ? -(x) : (x))
#define ll long long
#define ld long double
#define all(a) a.begin(), a.end()
#define pii pair<ll, int>
#define forn(i, n) for(int i = 0; i < n; i++)
#ifdef DEBUG
	#define NAME "1"
#else
	#define NAME "landscape"
#endif
#define FREOPEN freopen(NAME".in", "r", stdin); freopen(NAME".out", "w", stdout)
#define PI 3.1415926535897932384626433832795
//#define y second
//#define x first
ll n, amount, w[MAXN], pr_sum[MAXN + 1];
ll ans = 0;
set<pii> p1, p2;

int main() {
	ios_base::sync_with_stdio(0);
	FREOPEN;
	cin >> n >> amount;
	forn(i, n)
		cin >> w[i];
	pr_sum[0] = 0;
	forn(i, n)
		pr_sum[i + 1] = pr_sum[i] + w[i];
	forn(i, n)
		p2.insert(pii(w[i] + i, i));
	forn(i, n) {
		auto it = pii(w[i] + i, i);
		p2.erase(it);
		it = pii(w[i] - i, i);
		if(i == 0 || i == n - 1) {
			ans = max(ans, w[i]);
			p1.insert(it);
			continue;
		}
		ll l = w[i], r = 2e18;
		while(r - l > 1) {
			ll h = (l + r) / 2;
			auto it_l = p1.lower_bound(pii(h - i, -1));
			auto it_r = p2.lower_bound(pii(h + i, -1));
			bool fl = 1;
			if(it_l == p1.end() || it_r == p2.end()) {
				fl = 0;
			}
			else {
			   int j_l = it_l->second;
			   j_l++;
			   int j_r = it_r->second;
			   j_r--;
			   ll need = 0;
			   need += max(0LL, ((h + h - ABS(i - j_l)) * (ABS(i - j_l) + 1) / 2) - (pr_sum[i + 1] - pr_sum[j_l]));
			   need += max(0LL, ((h + h - ABS(i - j_r)) * (ABS(i - j_r) + 1) / 2) - (pr_sum[j_r + 1] - pr_sum[i]));
			   need -= (h - w[i]);
			   fl = (need <= amount);
			}
			if(fl)
				l = h;
			else
				r = h;
		}
		ans = max(ans, l);
		p1.insert(it);
	}
	cout << ans;
	//#ifdef DEBUG
  	//	cout << "\n\nTIME ELAPSED: " << ((float)clock()) / CLOCKS_PER_SEC << "\n";
	//#endif
	return 0;
}