#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <math.h>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <math.h>
#include <iostream> 
#include <random>
#include<map>
#include <iomanip>
#include <time.h>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
#include <cassert>
#include<fstream>
#include <unordered_map>  
using namespace std;
#define eps 0.00000001
#define LONG_INF 10000000000000000
#define GOLD 1.61803398874989484820458
#define MAX_MOD 1000000007
#define REP(i,n) for(long long i = 0;i < n;++i)
long long ans[200][3] = {};
int main() {
	long long n, d;
	cin >> n >> d;
	ans[0][1] = -1 * d;
	for (int i = 0;i < n;++i) {
		long long a, b;
		cin >> a >> b;
		ans[i + 1][0] = max(ans[i][0] + a, ans[i][1] + a - d);
		ans[i + 1][1] = max(ans[i][1] + b, ans[i][0] + b - d);
	}
	cout << max(ans[n][0], ans[n][1]) << endl;
	return 0;
}