#include<bits/stdc++.h>
using namespace std;
#define int long long
#define vi vector<int>
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
#define pi 3.141592653589793238
#define rep(i,a,b) for (int i = a; i <= b; i++)
#define zip(i,a,b) for(int i=a;i<b;i++)
#define rzip(i,a,b) for(int i=a;i>=b;i--)
#define ll unsigned int
#define test int t;cin>>t; while(t--)
#define en '\n'
typedef pair<int, int>     pii;
typedef pair<char, int>    pci;
typedef pair<char, char>   pcc;
typedef vector<pii>       vii;
typedef long double ld;
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(),v.rend()
#define sz(x) (int)x.size()
#define INF (1e18+5)
#define inf (1e9+5)
#define mod 1000000007


void __print(int x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
// You should only debug a pair of simple data types. For example,
// the debug won't work if one of pair's elements is collection type
// (std::vector, std::map, std::set...).
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i : x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifndef ONLINE_JUDGE
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif


// bool check_prime(int n)  return whether a number is prime or not O(sqrt(N));
// int BE(int x,int n,int m)  return x^n%m; O(logN);
// void sieve()  from number 1-1000001 saare prime store kar lega  in bool is_prime[1000001] array
// vector<int> z_array(string s)  return vector which is Z-array of string s;
// vector<int> lps(string s)      return vector which is lps array of string s;
// int power(int x,int n) return x^n; O(logN);
// using_ordered_set (template for using ordered set , replace first 2 lines of this page with this code..);
const int gf = 1e6 + 9;


int solve(int x[], int v[], int n, int c)
{
	int ans = 0; // to be returned.......

	// circumference of cirlce = c;
	// we start form 0
	vi pre(n, 0);
	pre[0] = v[0] - x[0];
	debug(pre[0]);
	ans = max(ans, pre[0]);
	//debug(ans);
	zip(i, 1, n)
	{
		pre[i] = pre[i - 1] - (x[i] - x[i - 1]) + v[i];
		debug(i, pre[i]);
		ans = max(ans, pre[i]);
	}
	///debug(ans);
	int mx[n];
	vi suff(n, 0);
	suff[n - 1] = v[n - 1] - (c - x[n - 1]);
	rzip(i, n - 2, 0)
	{
		suff[i] = suff[i + 1] - (x[i + 1] - x[i]) + v[i];
	}
	debug(suff);
	mx[n - 1] = suff[n - 1];
	int nikal = suff[n - 1];
	rzip(i, n - 2, 0)
	{
		// max upto i th index
		nikal = max(nikal, suff[i]);
		mx[i] = nikal;
	}
	debug(mx[2]);
	debug(pre[1]);
	zip(i, 0, n - 1)
	{
		ans = max(ans, pre[i] - x[i] + mx[i + 1]);
	}
	return ans;
}


int32_t main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	cout.tie(0);
#ifndef ONLINE_JUDGE
	if (fopen("input.txt", "r"))
	{
		freopen ("input.txt" , "r" , stdin);
		freopen ("output.txt" , "w" , stdout);
	}
#endif
	//_______________________________-code starts-_______________________________________________

	int n, c;
	cin >> n >> c;
	int x[n], v[n];
	zip(i, 0, n)
	{
		cin >> x[i] >> v[i];
	}
	int ans = 0;
	ans = solve(x, v, n, c);
	//debug(ans);
	reverse(v, v + n);
	reverse(x, x + n);
	zip(i, 0, n)
	{
		x[i] = c - x[i];
	}
	ans = max(ans, solve(x, v, n, c));
	//debug(solve(x, v, n, c));
	cout << ans << en;

	return 0;
}




Measured memory_bytes: 7156