#include <iostream>
#include <vector>
using namespace std;
int main(void) {
	long long n, d, t, k;
	cin >> n >> d;
	vector<vector<long long>> 
		dp(n + 1, vector<long long>(2, 0));
	dp[0][1] = -d;
	for (int i = 1; i < n + 1; ++i) {
		cin >> t >> k;
		dp[i][0] = 
			max(dp[i - 1][0] + t, dp[i - 1][1] + t - d);
		dp[i][1] =
			max(dp[i - 1][0] + k - d, dp[i - 1][1] + k);
	}
	cout << max(dp[n][0], dp[n][1]) << endl;
	return 0;
}