using System;
using System.IO;
using System.Collections.Generic;
using System.Linq;
using System.Numerics;
using System.Security.Principal;

namespace AtCoder.Contest.E
{
	static class Program
	{
		public static void Solve(Scanner cin)
		{
			var (n, c) = cin.ReadValue<int, long>();
			var (_x, _v) = cin.ReadValueArray<long, long>(n);
			var x = new List<long>();
			var v = new List<long>();
			x.Add(0); v.Add(0);
			for (int i = 0; i < n; i++)
			{
				x.Add(_x[i]); v.Add(_v[i]);
			}
			x.Add(c); v.Add(0);

			var sumleft = new long[n + 2];
			var sumright = new long[n + 2];
			for (int i = 0; i < n + 2; i++)
			{
				if (i == 0)
				{
					sumleft[i] = v[i];
					sumright[i] = v[n + 1 - i];
				}
				else
				{
					sumleft[i] = v[i] + sumleft[i - 1];
					sumright[i] = v[n + 1 - i] + sumright[i - 1];
				}
			}
			var l1 = new long[n + 2];
			var l2 = new long[n + 2];
			var r1 = new long[n + 2];
			var r2 = new long[n + 2];

			for (int i = 0; i < n + 2; i++)
			{
				l1[i] = sumleft[i] - x[i];
				l2[i] = sumleft[i] - x[i] * 2;
				r1[i] = sumright[i] - (c - x[n + 1 - i]);
				r2[i] = sumright[i] - (c - x[n + 1 - i]) * 2;
			}

			var maxl1 = new long[n + 2];
			var maxl2 = new long[n + 2];
			var maxr1 = new long[n + 2];
			var maxr2 = new long[n + 2];

			for (int i = 0; i < n + 2; i++)
			{
				if (i == 0)
				{
					maxl1[i] = l1[i];
					maxl2[i] = l2[i];
					maxr1[i] = r1[i];
					maxr2[i] = r2[i];
				}
				else
				{
					maxl1[i] = Math.Max(maxl1[i - 1], l1[i]);
					maxl2[i] = Math.Max(maxl2[i - 1], l2[i]);
					maxr1[i] = Math.Max(maxr1[i - 1], r1[i]);
					maxr2[i] = Math.Max(maxr2[i - 1], r2[i]);
				}				
			}

			var l1s = new SegmentTree(l1);
			var l2s = new SegmentTree(l2);
			var r1s = new SegmentTree(r1);
			var r2s = new SegmentTree(r2);

			long ans = 0;

			for (int i = 1; i < n + 1; i++)
			{
				//ans = Math.Max(ans, l1[i] + r2s.GetMax(0, n - i));
				//ans = Math.Max(ans, r1[i] + l2s.GetMax(0, n - i));

				ans = Math.Max(ans, l1[i] + maxr2[n - i]);
				ans = Math.Max(ans, r1[i] + maxl2[n - i]);
			}

			Console.WriteLine(ans);
		}

		public static void Main(string[] args)
		{
			var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
			Console.SetOut(sw);
			var cin = new Scanner();
			Solve(cin);
			Console.Out.Flush();
		}

		public static void YESNO(bool condition)
		{
			if (condition) Console.WriteLine("YES");
			else Console.WriteLine("NO");
		}

		public static void YesNo(bool condition)
		{
			if (condition) Console.WriteLine("Yes");
			else Console.WriteLine("No");
		}

		public static void yesno(bool condition)
		{
			if (condition) Console.WriteLine("yes");
			else Console.WriteLine("no");
		}
	}

	class SegmentTree
	{
		// 元の配列の大きさ
		public int N { get; set; }
		// セグメント木のデータ(2^n-1)
		public long[] Node { get; set; }

		// 元データの配列を引数で渡す
		public SegmentTree(long[] a)
		{
			// 要素数を2のべき乗とする
			N = 1;
			while (N < a.Length) N *= 2;

			// セグメント木の管理する要素数を 2^n-1 で初期化する
			this.Node = new long[2 * N - 1];
			for (int i = 0; i < this.Node.Length; i++) this.Node[i] = -1;

			// 各ノードの値を初期化する
			// 葉は元の配列のデータ
			for (int i = 0; i < a.Length; i++) this.Node[N + i - 1] = a[i];
			// 子ノードを比較して親ノードの値を決めていく
			for (int i = N - 2; i >= 0; i--)
			{
				this.Node[i] = this.Compare(this.Node[2 * i + 1], this.Node[2 * i + 2]);
			}
		}

		// x番目のデータをvに更新する
		public void Update(int x, long v)
		{
			// 元の配列のデータが格納される葉のデータを更新する
			x += (this.N - 1);
			this.Node[x] = v;

			// 更新したノードの親をさかのぼってルートまで更新する
			while (x > 0)
			{
				// 親ノードのインデックス
				x = (x - 1) / 2;
				this.Node[x] = this.Compare(this.Node[2 * x + 1], this.Node[2 * x + 2]);
			}
		}
		// [a, b) の区間最小値を求める
		public long GetMax(int a, int b)
		{
			return this.GetMax(a, b, 0, 0, this.N);
		}
		// [a, b) の区間最小値を求める
		// 現在のノードはk番目、[l, r)
		private long GetMax(int a, int b, int k, int l, int r)
		{
			// 1. 現在のノードの対応区間が、求める区間に含まれない場合
			//    対象外の区間のデータなので、最小値に関係のない値(INF)を返す
			if (r <= a || b <= l) return -1;

			// 2. 現在のノードの対応区間が、求める区間に完全に含まれる場合
			//    現在のノードの値が最小値の候補になるのでそのまま返す
			if (a <= l && r <= b) return this.Node[k];

			// 3. 上記以外の場合、現在のノードの対応区間が、求める区間を一部を含む場合
			//    子ノードを評価し、比較した結果を返す
			var vl = this.GetMax(a, b, 2 * k + 1, l, (l + r) / 2);
			var vr = this.GetMax(a, b, 2 * k + 2, (l + r) / 2, r);
			return this.Compare(vl, vr);
		}

		// 2要素を比較して小さい値を返す
		private long Compare(long a, long b) { return Math.Max(a, b); }
	}

	class Scanner
	{
		string[] s;
		int i;

		char[] separator = new char[] { ' ' };

		public Scanner()
		{
			s = new string[0];
			i = 0;
		}

		public string Read() => ReadString();

		public string ReadString()
		{
			if (i < s.Length) return s[i++];
			string st = Console.ReadLine();
			while (st == "") st = Console.ReadLine();
			s = st.Split(separator, StringSplitOptions.RemoveEmptyEntries);
			if (s.Length == 0) return ReadString();
			i = 0;
			return s[i++];
		}

		public string[] ReadStringArray(int N)
		{
			string[] Array = new string[N];
			for (int i = 0; i < N; i++)
			{
				Array[i] = ReadString();
			}
			return Array;
		}

		public int ReadInt()
		{
			return int.Parse(ReadString());
		}

		public int[] ReadIntArray(int N, int add = 0)
		{
			int[] Array = new int[N];
			for (int i = 0; i < N; i++)
			{
				Array[i] = ReadInt() + add;
			}
			return Array;
		}

		public long ReadLong()
		{
			return long.Parse(ReadString());
		}

		public long[] ReadLongArray(int N, long add = 0)
		{
			long[] Array = new long[N];
			for (int i = 0; i < N; i++)
			{
				Array[i] = ReadLong() + add;
			}
			return Array;
		}

		public double ReadDouble()
		{
			return double.Parse(ReadString());
		}

		public double[] ReadDoubleArray(int N, double add = 0)
		{
			double[] Array = new double[N];
			for (int i = 0; i < N; i++)
			{
				Array[i] = ReadDouble() + add;
			}
			return Array;
		}

		public T1 ReadValue<T1>() => (T1)Convert.ChangeType(ReadString(), typeof(T1));

		public (T1, T2) ReadValue<T1, T2>()
		{
			var inputs = ReadStringArray(2);
			var v1 = (T1)Convert.ChangeType(inputs[0], typeof(T1));
			var v2 = (T2)Convert.ChangeType(inputs[1], typeof(T2));
			return (v1, v2);
		}

		public (T1, T2, T3) ReadValue<T1, T2, T3>()
		{
			var inputs = ReadStringArray(3);
			var v1 = (T1)Convert.ChangeType(inputs[0], typeof(T1));
			var v2 = (T2)Convert.ChangeType(inputs[1], typeof(T2));
			var v3 = (T3)Convert.ChangeType(inputs[2], typeof(T3));
			return (v1, v2, v3);
		}

		public (T1, T2, T3, T4) ReadValue<T1, T2, T3, T4>()
		{
			var inputs = ReadStringArray(4);
			var v1 = (T1)Convert.ChangeType(inputs[0], typeof(T1));
			var v2 = (T2)Convert.ChangeType(inputs[1], typeof(T2));
			var v3 = (T3)Convert.ChangeType(inputs[2], typeof(T3));
			var v4 = (T4)Convert.ChangeType(inputs[3], typeof(T4));
			return (v1, v2, v3, v4);
		}

		public (T1, T2, T3, T4, T5) ReadValue<T1, T2, T3, T4, T5>()
		{
			var inputs = ReadStringArray(5);
			var v1 = (T1)Convert.ChangeType(inputs[0], typeof(T1));
			var v2 = (T2)Convert.ChangeType(inputs[1], typeof(T2));
			var v3 = (T3)Convert.ChangeType(inputs[2], typeof(T3));
			var v4 = (T4)Convert.ChangeType(inputs[3], typeof(T4));
			var v5 = (T5)Convert.ChangeType(inputs[4], typeof(T5));
			return (v1, v2, v3, v4, v5);
		}

		public (T1, T2, T3, T4, T5, T6) ReadValue<T1, T2, T3, T4, T5, T6>()
		{
			var inputs = ReadStringArray(6);
			var v1 = (T1)Convert.ChangeType(inputs[0], typeof(T1));
			var v2 = (T2)Convert.ChangeType(inputs[1], typeof(T2));
			var v3 = (T3)Convert.ChangeType(inputs[2], typeof(T3));
			var v4 = (T4)Convert.ChangeType(inputs[3], typeof(T4));
			var v5 = (T5)Convert.ChangeType(inputs[4], typeof(T5));
			var v6 = (T6)Convert.ChangeType(inputs[5], typeof(T6));
			return (v1, v2, v3, v4, v5, v6);
		}

		public (T1, T2, T3, T4, T5, T6, T7) ReadValue<T1, T2, T3, T4, T5, T6, T7>()
		{
			var inputs = ReadStringArray(7);
			var v1 = (T1)Convert.ChangeType(inputs[0], typeof(T1));
			var v2 = (T2)Convert.ChangeType(inputs[1], typeof(T2));
			var v3 = (T3)Convert.ChangeType(inputs[2], typeof(T3));
			var v4 = (T4)Convert.ChangeType(inputs[3], typeof(T4));
			var v5 = (T5)Convert.ChangeType(inputs[4], typeof(T5));
			var v6 = (T6)Convert.ChangeType(inputs[5], typeof(T6));
			var v7 = (T7)Convert.ChangeType(inputs[6], typeof(T7));
			return (v1, v2, v3, v4, v5, v6, v7);
		}

		public T1[] ReadValueArray<T1>(int N)
		{
			var v1 = new T1[N];
			for (int i = 0; i < N; i++)
			{
				v1[i] = ReadValue<T1>();
			}
			return v1;
		}

		public (T1[], T2[]) ReadValueArray<T1, T2>(int N)
		{
			var (v1, v2) = (new T1[N], new T2[N]);
			for (int i = 0; i < N; i++)
			{
				var (t1, t2) = ReadValue<T1, T2>();
				v1[i] = t1;
				v2[i] = t2;
			}
			return (v1, v2);
		}

		public (T1[], T2[], T3[]) ReadValueArray<T1, T2, T3>(int N)
		{
			var (v1, v2, v3) = (new T1[N], new T2[N], new T3[N]);
			for (int i = 0; i < N; i++)
			{
				var (t1, t2, t3) = ReadValue<T1, T2, T3>();
				v1[i] = t1;
				v2[i] = t2;
				v3[i] = t3;
			}
			return (v1, v2, v3);
		}

		public (T1[], T2[], T3[], T4[]) ReadValueArray<T1, T2, T3, T4>(int N)
		{
			var (v1, v2, v3, v4) = (new T1[N], new T2[N], new T3[N], new T4[N]);
			for (int i = 0; i < N; i++)
			{
				var (t1, t2, t3, t4) = ReadValue<T1, T2, T3, T4>();
				v1[i] = t1;
				v2[i] = t2;
				v3[i] = t3;
				v4[i] = t4;
			}
			return (v1, v2, v3, v4);
		}

		public (T1[], T2[], T3[], T4[], T5[]) ReadValueArray<T1, T2, T3, T4, T5>(int N)
		{
			var (v1, v2, v3, v4, v5) = (new T1[N], new T2[N], new T3[N], new T4[N], new T5[N]);
			for (int i = 0; i < N; i++)
			{
				var (t1, t2, t3, t4, t5) = ReadValue<T1, T2, T3, T4, T5>();
				v1[i] = t1;
				v2[i] = t2;
				v3[i] = t3;
				v4[i] = t4;
				v5[i] = t5;
			}
			return (v1, v2, v3, v4, v5);
		}

		public (T1[], T2[], T3[], T4[], T5[], T6[]) ReadValueArray<T1, T2, T3, T4, T5, T6>(int N)
		{
			var (v1, v2, v3, v4, v5, v6) = (new T1[N], new T2[N], new T3[N], new T4[N], new T5[N], new T6[N]);
			for (int i = 0; i < N; i++)
			{
				var (t1, t2, t3, t4, t5, t6) = ReadValue<T1, T2, T3, T4, T5, T6>();
				v1[i] = t1;
				v2[i] = t2;
				v3[i] = t3;
				v4[i] = t4;
				v5[i] = t5;
				v6[i] = t6;
			}
			return (v1, v2, v3, v4, v5, v6);
		}

		public (T1[], T2[], T3[], T4[], T5[], T6[], T7[]) ReadValueArray<T1, T2, T3, T4, T5, T6, T7>(int N)
		{
			var (v1, v2, v3, v4, v5, v6, v7) = (new T1[N], new T2[N], new T3[N], new T4[N], new T5[N], new T6[N], new T7[N]);
			for (int i = 0; i < N; i++)
			{
				var (t1, t2, t3, t4, t5, t6, t7) = ReadValue<T1, T2, T3, T4, T5, T6, T7>();
				v1[i] = t1;
				v2[i] = t2;
				v3[i] = t3;
				v4[i] = t4;
				v5[i] = t5;
				v6[i] = t6;
				v7[i] = t7;
			}
			return (v1, v2, v3, v4, v5, v6, v7);
		}
	}
}


Measured memory_bytes: 64024