#include <iostream>
#include <algorithm>
#include <string>
#include <cstdio>
#include <cmath>
#include <stack>
#include <cstring>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <numeric>


int main()
{
	long long n, d;

	std::cin >> n >> d;

	// tokyo, kyoto
	std::vector<std::pair<long long, long long> > v(n);

	for (int i = 0; i < n; i++) {
		long long a, b;

		std::cin >> a >> b;

		v[i] = { a, b };
	}

	long long tokyo = v[0].first;
	long long kyoto = v[0].second - d;

	for (size_t i = 1; i < n; i++) {
		long long tmp = tokyo;
		tokyo = std::max(tokyo + v[i].first, kyoto - d + v[i].first);
		kyoto = std::max(kyoto + v[i].second, tmp - d + v[i].second);
	}

	std::cout << std::max(tokyo, kyoto) << std::endl;

	return 0;
}