#include <algorithm>
#include <iostream>
#include <utility>
#include <vector>
using namespace std;

int main() {
	int D;
	size_t N;
	cin >> N >> D;
	vector<pair<int, int>> TK(N), dp(N);
	for (auto& x : TK) cin >> x.first >> x.second;
	dp.at(0).first = TK.at(0).first;
	dp.at(0).second = TK.at(0).second - D;
	for (size_t i = 1; i < N; ++i) {
		dp.at(i).first = max(dp.at(i - 1).second - D, dp.at(i - 1).first) + TK.at(i).first;
		dp.at(i).second = max(dp.at(i - 1).first - D, dp.at(i - 1).second) + TK.at(i).second;
	}
	cout << max(dp.at(N - 1).first, dp.at(N - 1).second);
}