#include <bits/stdc++.h>

using namespace std;

typedef long long LL;
typedef pair<int, int> PII;

const int N = 2e5 + 5, M = 8e5 + 5;

int n, m, a[N], b[N];
struct Frac{
	LL a, b;
	Frac() {}
	Frac(LL _a, LL _b) : a(_a), b(_b) {}
	inline bool operator <(const Frac &w) const
	{
		return (__int128)a * w.b < (__int128)b * w.a;
	}
	inline bool operator ==(const Frac &w) const
	{
		return (__int128)a * w.b == (__int128)b * w.a;
	}
};
struct Block{
	Frac val;
	LL s;
};
Block sta[N];
int tt;
Frac xx[M];
int ss;
vector<Block> dl[N], dr[N];
struct Node{
	int a, b;
	LL cnt, sum, v;
}t[M * 4];
int pos[M];
LL sum, ans;

inline void Get()
{
	for(int i = 1; i <= n; i ++ )
	{
		LL s = b[i], c = 1, x = b[i];
		while(tt && Frac(s, c) < sta[tt].val)
		{
			x += s * sta[tt].val.b + sta[tt].s;
			s += sta[tt].val.a, c += sta[tt].val.b;
			dl[i].push_back({sta[tt].val, -sta[tt].s});
			tt -- ;
		}
//		printf("!!%lld %lld %lld\n", s, c, x);
		sta[ ++ tt] = /**/ {{s, c}, x};
		xx[ ++ ss] = /**/ {s, c};
		dl[i].push_back(sta[tt]);
//		printf("#%d\n", i);
//		for(int i = 1; i <= tt; i ++ ) printf("(%lld %lld %lld) ", sta[i].val.a, sta[i].val.b, sta[i].s);
//		puts("");
	}
	tt = 0;
	for(int i = n + 1; i > 1; i -- )
	{
		LL s = b[i], c = 1, x = b[i];
		while(tt && Frac(s, c) < sta[tt].val)
		{
//			printf("**%lld %lld %lld\n", sta[tt].val.a, sta[tt].val.b, s);
			x += s * sta[tt].val.b + sta[tt].s;
			s += sta[tt].val.a, c += sta[tt].val.b;
			dr[i].push_back(sta[tt]);
			tt -- ;
		}
//		printf("*%d %lld\n", i, x);
		sta[ ++ tt] = /**/ {{s, c}, x};
		xx[ ++ ss] = /**/ {s, c};
		dr[i].push_back({sta[tt].val, -sta[tt].s});
	}
}

inline void Hash()
{
	sort(xx + 1, xx + ss + 1);
	ss = unique(xx + 1, xx + ss + 1) - xx - 1;
//	for(int i = 1; i <= ss; i ++ ) printf("(%lld %lld) ", xx[i].a, xx[i].b);
//	puts("");
}
inline int Find(Frac &x)
{
	return lower_bound(xx + 1, xx + ss + 1, x) - xx;
}

#define ls (u << 1)
#define rs (u << 1 | 1)
inline void PushUp(int u)
{
	t[u].cnt = t[ls].cnt + t[rs].cnt;
	t[u].sum = t[ls].sum + t[rs].sum;
	t[u].v = t[ls].v + t[rs].v + t[rs].sum * t[ls].cnt;
}
inline void Build(int u, int a, int b)
{
	t[u].a = a, t[u].b = b;
	if(a == b) pos[a] = u;
	else
	{
		int mid = (a + b) >> 1;
		Build(ls, a, mid);
		Build(rs, mid + 1, b);
	}
}
inline void Modify(int x, LL dc, LL ds)
{
	int u = pos[x];
	if(dc > 0) t[u].v += t[u].cnt * ds;
	t[u].cnt += dc, t[u].sum += ds;
	if(dc < 0) t[u].v += t[u].cnt * ds;
//	printf("!%d %lld %lld %lld\n", x, t[u].cnt, t[u].sum, t[u].v);
	for(u >>= 1; u; u >>= 1) PushUp(u);
}

inline void Work()
{
	Build(1, 1, ss);
	for(int i = 1; i <= tt; i ++ )
	{
		sum += sta[i].s;
		Modify(Find(sta[i].val), sta[i].val.b, sta[i].val.a);
//		printf("!!%lld %lld %lld\n", sta[i].val.a, sta[i].val.b, sta[i].s);
	}
	ans = sum + t[1].v + 1LL * n * b[1];
//	printf("#%lld %lld %lld\n", sum + t[1].v + 1LL * n * b[1], sum, t[1].v);
	for(int j = 2; j <= n + 1; j ++ )
	{
		for(Block &i : dr[j])
		{
//			printf("!%lld %lld %lld\n", i.val.a, i.val.b, i.s);
			sum += i.s;
			if(i.s > 0) Modify(Find(i.val), i.val.b, i.val.a);
			else Modify(Find(i.val), -i.val.b, -i.val.a);
		}
		for(Block &i : dl[j - 1])
		{
//			printf("!%lld %lld %lld\n", i.val.a, i.val.b, i.s);
			sum += i.s;
			if(i.s > 0) Modify(Find(i.val), i.val.b, i.val.a);
			else Modify(Find(i.val), -i.val.b, -i.val.a);
		}
		ans = max(ans, sum + t[1].v + 1LL * n * b[j]);
//		printf("#%lld %lld %lld\n", sum + t[1].v + 1LL * n * b[j], sum, t[1].v);
	}
}

int main()
{
	scanf("%d%d", &n, &m);
	for(int i = 1; i <= n; i ++ ) scanf("%d", &a[i]);
	sort(a + 1, a + n + 1);
	for(int i = 1; i <= n; i ++ ) b[i] = a[i] - a[i - 1];
	b[n + 1] = m + 1 - a[n];
	
	Get(), Hash(), Work();
	
	printf("%lld\n", ans);
	return 0;
}