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

int main() {
	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(101, vector<int>(3));
	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] + t[i], dp[i - 1][1] + t[i] - d);
		dp[i][1] = max(dp[i - 1][0] + k[i] - d, dp[i - 1][1] + k[i]);
	}
	cout << max(dp[n - 1][0], dp[n - 1][1]);
}