#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <iomanip>
#include <stdio.h>
#include <numeric>
#include <math.h>
#include <algorithm>
#include <functional>
#include <string>
#include <array>
#include <vector>
#include <list>
#include <stack>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <deque>
#include <queue>
#include <complex>
#include <bitset>
#define full(c) c.begin(), c.end()
#define vector2d(name,type,h,w,init) vector<vector<type>> name(h,vector<type>(w,init)) 
const int PRIME = 1000000007;
const int INT_INF = 2147483647;
const long long int LL_INF = 9223372036854775807;
const double PI = acos(-1);
const double EPS = 0.0001;
typedef unsigned long long int ull;
typedef long long int ll;
using namespace std;

int main() {
	//cin.tie(0);
	//ios::sync_with_stdio(false);
	ll n, d, mt = 0, mk = 0;
	cin >> n >> d;
	mk -= d;
	for (int i = 0; i < n; i++) {
		ll t, k, nt, nk;
		cin >> t >> k;
		nt = max(mt + t, mk + k - d);
		nk = max(mt + t - d, mk + k);
		mt = nt;
		mk = nk;
	}
	cout << max(mt, mk) << endl;
	return 0;
}