#include<cstdio>
#include<algorithm>
#include<utility>
#include<vector>
#define mp make_pair
using namespace std;
const int max_n = 100100;
typedef pair<int,int> pi;
typedef long long ll;
int n,l;
ll ans,dp1[max_n],dp2[max_n];
pi p[max_n];
vector<int> up[max_n], down[max_n], v1, v2;
void input() {
scanf("%d%d",&n,&l);
for(int i=0;i<n;i++) scanf("%d%d",&p[i].first,&p[i].second);
for(int i=0;i<n;i++) v1.push_back(p[i].first);
sort(v1.begin(),v1.end());
v1.erase(unique(v1.begin(),v1.end()),v1.end());
for(int i=0;i<n;i++) {
int lo = 0, hi = (int)v1.size()-1;
while(lo < hi) {
int mid = (lo+hi-1)/2;
if(v1[mid] < p[i].first) lo = mid+1;
else hi = mid;
}
p[i].first = lo;
}
for(int i=0;i<n;i++) v2.push_back(p[i].second);
sort(v2.begin(), v2.end());
v2.erase(unique(v2.begin(), v2.end()), v2.end());
for(int i=0;i<n;i++) {
int lo = 0, hi = (int)v2.size()-1;
while(lo < hi) {
int mid = (lo+hi-1)/2;
if(v2[mid] < p[i].second) lo = mid+1;
else hi = mid;
}
p[i].second = lo;
}
for(int i=0;i<n;i++) {
up[p[i].first].push_back(i);
down[p[i].second].push_back(i);
}
return;
}
inline int abs(int x) {return x>0?x:-x;}
void solve() {
sort(p,p+n);
for(int i=n-1;i>=0;i--) {
ll dist = 1LL*abs(v1[p[i].first]-v2[p[i].second]) + l;
ll dp1_ = dp2[p[i].second] + dist;
ll dp2_ = dp1[p[i].first] + dist;
dp1[p[i].first] = max(dp1[p[i].first], dp1_);
dp2[p[i].second] = max(dp2[p[i].second], dp2_);
}
for(int i=0;i<n;i++) {
ans = max(ans, dp1[i]);
ans = max(ans, dp2[i]);
}
printf("%lld",ans);
return;
}
int main() {
input();
solve();
return 0;
}
