#include <iostream>
#include <algorithm>

using namespace std;

int main() {
	long long int dp[2][105], n, d;
	char where = 't';
	cin >> n >> d;
	for (int i = 1; i <= n; i++) {
		long long int t, k;
		cin >> t >> k;
		if (i == 1) {
			dp[0][i] = t;
			dp[1][i] = k - d;
		}
		else {
			dp[0][i] = max(dp[0][i - 1] + t, dp[1][i - 1] + t - d);
			dp[1][i] = max(dp[1][i - 1] + k, dp[0][i - 1] + k - d);
		}
	}
	cout << max(dp[0][n], dp[1][n]) << endl;
	return 0;
}