#include<iostream>
#include<string>
#include<string.h>
#include<algorithm>
#include<stdio.h>
#include<cmath>
#include<vector>
#include<utility>
#include<stack>
#include<queue>
#include<list>
#include<bitset>

#define FOR(i, a, b) for(int i=(a);i<=(b);i++)
#define RFOR(i, a, b) for(int i=(a);i>=(b);i--)
#define MOD 1000000007
#define INF 1000000000

using namespace std;


int main(void) {
	int n, d;
	int t[101], k[101];
	int dp[101][3] = {};

	cin >> n >> d;
	FOR(i, 1, n) {
		cin >> t[i] >> k[i];
	}
	dp[1][1] = t[1];
	dp[1][2] = k[1]-d;
	FOR(i, 2, n) {
		dp[i][1] = max(dp[i - 1][1] + t[i], dp[i - 1][2] - d + t[i]);
		dp[i][2] = max(dp[i - 1][1] - d + k[i], dp[i - 1][2] + k[i]);
	}
	cout << max(dp[n][1], dp[n][2]) << endl;
	return 0;
}