#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<ctime>
#include<cctype>
#include<climits>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<memory>
#include<functional>
#include<set>
 
using namespace std;
 
#define ALL(g) (g).begin(),(g).end()
#define REP(i, x, n) for(int i = x; i < n; i++)
#define rep(i,n) REP(i,0,n)
#define F(i,j,k) fill(i[0],i[0]+j*j,k)
#define P(p) cout<<(p)<<endl;
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define INF 1<<25
#define pb push_back
 
typedef vector<int> vi;
typedef vector<long long> vl;
typedef vector<double> vd;
typedef pair<int,int> pii;
typedef pair<long,long> pll;
typedef long long ll;
int t[101],k[101];
int n,d,ans;

void dfs(int day,int nx_place,int yen){
	//	cout<<yen<<endl;
	if(day==n+1){
		ans=max(ans,yen);
		return ; 
	}
	if(nx_place==0)
		yen+=t[day];
	else
		yen+=k[day];

	//stay
		dfs(day+1,nx_place,yen);
	//mov
	if(nx_place==0)
		dfs(day+1,1,yen-d);
	else
		dfs(day+1,0,yen-d);
}
int main(){
	cin>>n>>d;
	rep(i,n){
		cin>>t[i]>>k[i];
	}
	dfs(0,0,0);
	dfs(0,1,-d);
	cout<<ans<<endl;
	return 0;
}