#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <ctime>
#include <cassert>
#include <string>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <functional>
#include <iostream>
#include <map>
#include <set>
#include <cassert>
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> P;
typedef pair<int,P> P1;
typedef pair<P,P> P2;
#define pu push
#define pb push_back
#define mp make_pair
#define eps 1e-7
#define INF 1000
#define mod 1000000007
#define fi first
#define sc second
#define rep(i,x) for(int i=0;i<x;i++)
#define SORT(x) sort(x.begin(),x.end())
#define ERASE(x) x.erase(unique(x.begin(),x.end()),x.end())
#define POSL(x,v) (lower_bound(x.begin(),x.end(),v)-x.begin())
#define POSU(x,v) (upper_bound(x.begin(),x.end(),v)-x.begin())
ll dp[105][2],d,a[105],b[105];
int n;
int main(){
	cin >> n >> d;
	for(int i=0;i<n;i++) cin>>a[i]>>b[i];
	rep(i,105)rep(j,2)dp[i][j]=-1e18;
	dp[0][0] = 0;
	for(int i=0;i<n;i++){
		if(dp[i][0]<0)goto nxt;
		dp[i+1][0] = max(dp[i+1][0],dp[i][0]+a[i]);
		dp[i+1][1] = max(dp[i+1][1],dp[i][0]+b[i]-d);
		nxt:;
		if(dp[i][1]<0)goto nxt2;
		dp[i+1][0] = max(dp[i+1][0],dp[i][1]+a[i]-d);
		dp[i+1][1] = max(dp[i+1][1],dp[i][1]+b[i]);
		nxt2:;
	}
	cout << max(dp[n][0],dp[n][1]) << endl;
}