#include <bits/stdc++.h>

using namespace std;

using ll = long long;
using ul = unsigned long;

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);

	int n, d;
	cin >> n >> d;

	vector<int> t(n), k(n);
	for (int i = 0; i < n; ++i)
		cin >> t[i] >> k[i];

	vector<vector<int> > dp(n, vector<int>(2));
	dp[0][0] = t[0];
	dp[0][1] = k[0] - d;

	for (int i = 1; i < n; ++i)
	{
		dp[i][0] = max(dp[i - 1][0], dp[i - 1][1] - d) + t[i];
		dp[i][1] = max(dp[i - 1][0] - d, dp[i - 1][1]) + k[i];
	}

	cout << max(dp[n - 1][0], dp[n - 1][1]) << "\n";

	return 0;
}