#include <bits/stdc++.h>

using namespace std;

using ll = long long;

const ll INF = 1e10;

ll dp[105][2];

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	int n;
	ll d;
	cin >> n >> d;
	vector<ll> t(n), k(n);
	for (int i = 0; i < n; i++) {
		cin >> t[i] >> k[i];
	}

	for (int i = 0; i <= n; i++) {
		dp[i][0] = -INF;
		dp[i][1] = -INF;
	}

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