#include<iostream>
#include<algorithm>
#include<vector>
using namespace std;
typedef long long ll;
ll n,L;
struct line{
ll u,v;
line(int uu=0,int vv=0){
u=uu,v=vv;
}
}UP[100010],DOWN[100010];
vector<int> up[200010],down[200010];
bool cmp(line x,line y){
return x.u!=y.u?x.u<y.u:x.v<y.v;
}
bool cmp2(line x,line y){
return x.v!=y.v?x.v<y.v:x.u<y.u;
}
int lu[200010],sz;
ll d[100010][2];
ll f(int x,int k){
ll &ret=d[x][k];
if(ret)return ret;
int lo=0,hi=0,mid,z;
if(k==0){
//cout<<"x=("<<UP[x].u<<", "<<UP[x].v<<") k= "<<k<<endl;
hi=up[UP[x].u].size();
while(lo+1<hi){
mid=(lo+hi)/2;
if(DOWN[up[UP[x].u][mid]].v>UP[x].v)hi=mid;
else lo=mid;
}
for(int i=hi;i<up[UP[x].u].size();i++){
z=up[UP[x].u][i];
ret=max(ret,f(z,1));
break;
}
ret+=abs(lu[UP[x].v]-lu[UP[x].u])+L;
lo=0;
hi=down[UP[x].v].size();
while(lo+1<hi){
mid=(lo+hi)/2;
if(UP[down[UP[x].v][mid]].u>UP[x].u)hi=mid;
else lo=mid;
}
//for(int i=0;i<down[UP[x].v].size();i++)cout<<UP[down[UP[x].v][i]].u<<","<<UP[down[UP[x].v][i]].v<<" ";
//cout<<endl;
if(hi<down[UP[x].v].size())ret=max(ret,f(down[UP[x].v][hi],k));
}
else{
//cout<<"x=("<<DOWN[x].u<<", "<<DOWN[x].v<<") k= "<<k<<endl;
hi=down[DOWN[x].v].size();
while(lo+1<hi){
mid=(lo+hi)/2;
if(UP[down[DOWN[x].v][mid]].u>DOWN[x].u)hi=mid;
else lo=mid;
}
for(int i=hi;i<down[DOWN[x].v].size();i++){
z=down[DOWN[x].v][i];
ret=max(ret,f(z,0));
}
ret+=abs(lu[DOWN[x].v]-lu[DOWN[x].u])+L;
lo=0;
hi=up[DOWN[x].u].size();
while(lo+1<hi){
mid=(lo+hi)/2;
if(DOWN[up[DOWN[x].u][mid]].v>DOWN[x].v)hi=mid;
else lo=mid;
}
if(hi<up[DOWN[x].u].size())ret=max(ret,f(up[DOWN[x].u][hi],k));
}
return ret;
}
int main(){
ios::sync_with_stdio(0);
cin>>n>>L;
ll x,y;
for(int i=0;i<n;i++){
cin>>x>>y;
lu[sz++]=x;
lu[sz++]=y;
UP[i]=line(x,y);
DOWN[i]=line(x,y);
}
sort(lu,lu+sz);
sz=unique(lu,lu+sz)-lu;
for(int i=0;i<n;i++){
UP[i].u=lower_bound(lu,lu+sz,UP[i].u)-lu;
UP[i].v=lower_bound(lu,lu+sz,UP[i].v)-lu;
DOWN[i]=UP[i];
}
sort(UP,UP+n,cmp);
sort(DOWN,DOWN+n,cmp2);
for(int i=0;i<n;i++){
down[UP[i].v].push_back(i);
up[DOWN[i].u].push_back(i);
}
ll ans=0;
for(int i=0;i<n;i++){
if(UP[i].u<=UP[down[UP[i].v][0]].u){
ans=max(ans,f(i,0));
}
if(DOWN[i].v<=DOWN[up[DOWN[i].u][0]].v){
ans=max(ans,f(i,1));
}
}
cout<<ans;
}
