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

int main() {
	int n, d;
	cin >> n >> d;
	int dp[2][110];//0:tokyo,1:kyoto,j日目iで働く
	dp[0][0] = 0;dp[1][0] = 0;
	for (int i = 0;i < n;i++) {
		int t, k;
		cin >> t >> k;
		dp[0][i + 1] = max(dp[0][i], dp[1][i] - d) + t;
		dp[1][i + 1] = max(dp[0][i] - d, dp[1][i]) + k;
	}
	cout << max(dp[0][n], dp[1][n]) << endl;
}