//Yukicoder
//Q561 Tokyo and kyoto


//二次元可変配列
//vector <vector <int>> mass;
//vector <vector <int>> memo;

//#include "stdafx.h"
#include <iostream>
#include <vector>
#include <list>//list
#include <set> //tree
#include <map> //連想配列
#include <unordered_set> //hash
#include <unordered_map> //hash
#include <algorithm>
#include <iomanip>

using namespace std;
typedef signed long long LL;


#define START (0)
#define RIGHT (1)
#define UP    (2)
#define LEFT  (3)
#define DOWN  (4)

#define DATA_MAX (1000000)
#define FMAX(a,b)  ((a)>(b)?(a):(b))
#define FMIX(a,b)  ((a)<(b)?(a):(b))


vector <vector <LL>> memo;
vector<LL> T;
vector<LL> K;
vector<LL> dp_k;
vector<LL> dp_t;

LL N, D;


int main()
{

	cin >> N;
	cin >> D;

	T.resize(110);
	K.resize(110);
	dp_t.resize(110);
	dp_k.resize(110);

	for (int i = 0; i < N; i++)
	{
		cin >> T[i];
		cin >> K[i];
	}


    //東京のいる場合の最大利益
	dp_t[0] = T[0];
   
	//京都にいる場合の最大利益
	dp_k[0] = K[0] - D;



	int n = 1;
	while (1)
	{
		dp_t[n] = T[n] + FMAX(dp_t[n-1], dp_k[n-1] - D);
		dp_k[n] = K[n] + FMAX(dp_k[n-1], dp_t[n-1] - D);

		n++;
		if (n >= N)
			break;
	}
	
	cout << FMAX(dp_t[n-1],dp_k[n-1]) << endl;
	
	//getchar();
	return 0; //end
}