#include<stdio.h>
#include<algorithm>
using namespace std;
struct pp{
int x,y;
}ar[110001];
int xy[220001];
long long ans1[220001]={0,},an;//위
long long ans2[220001]={0,};//아래
bool cmp(struct pp a,struct pp b){
if(a.x==b.x) return a.y<b.y;
return a.x<b.x;
}
long long mx(long long a,long long b){
if(a<b) return b;
return a;
}
long long ab(long long k){
if(k<0) return -k;
return k;
}
int main()
{
int n,l,i,j;
long long kx,ky;
scanf("%d %d",&n,&l);
for(i=1;i<=n;i++){
scanf("%d %d",&ar[i].x,&ar[i].y);
xy[i*2-1]=ar[i].x;
xy[i*2]=ar[i].y;
}
sort(xy+1,xy+2*n+1);
for(i=1;i<=n;i++){
j=lower_bound(xy+1,xy+2*n+1,ar[i].x)-xy;
ar[i].x=j;
j=lower_bound(xy+1,xy+2*n+1,ar[i].y)-xy;
ar[i].y=j;
}
sort(ar+1,ar+n+1,cmp);
for(i=1;i<=n;i++){
kx=ans1[ar[i].x];
ky=ans2[ar[i].y];
ans1[ar[i].x]=mx(ky+ab(xy[ar[i].x]-xy[ar[i].y])+l,ans1[ar[i].x]);
ans2[ar[i].y]=mx(kx+ab(xy[ar[i].x]-xy[ar[i].y])+l,ans2[ar[i].y]);
}
for(i=1;i<=2*n;i++){
an=mx(an,mx(ans1[i],ans2[i]));
}
printf("%lld",an);
}
