#include<iostream>
#include<vector>
#include<algorithm>

using namespace std;

int main() {

	long n, d;
	cin >> n >> d;
	vector<vector<long>> dp;
	dp.resize(n+1);
	for (int i = 0;i < n + 1;i++) {
		dp[i].resize(2);
	}

	vector<long> t(n,0), k(n,0);

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

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

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

	return 0;
}