#include<bits/stdc++.h>
using namespace std;
typedef long long int lld;
typedef pair<int, int> pii;
int N, l;
vector<pii> S;
vector<int> U, L;
lld D[100005], E[100005];
inline int getx(int x) {
return lower_bound(U.begin(), U.end(), x) - U.begin();
}
inline int gety(int y) {
return lower_bound(L.begin(), L.end(), y) - L.begin();
}
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr); cout.tie(nullptr);
cin >> N >> l;
for(int i = 0; i < N; i++) {
int x, y;
cin >> x >> y;
S.push_back(pii(x, y));
U.push_back(x);
L.push_back(y);
}
sort(S.begin(), S.end());
sort(U.begin(), U.end());
sort(L.begin(), L.end());
U.erase(unique(U.begin(), U.end()), U.end());
L.erase(unique(L.begin(), L.end()), L.end());
lld rst = 0;
for(int i = 0; i < N; i++) {
int a = getx(S[i].first);
int b = gety(S[i].second);
lld ta, tb;
ta = max(D[a], E[b] + abs(S[i].first-S[i].second) + l);
tb = max(E[b], D[a] + abs(S[i].first-S[i].second) + l);
D[a] = ta;
E[b] = tb;
//cout << a << ' ' << D[a] << ' ';
//cout << b << ' ' << E[b] << endl;
}
for(int i = 0; i < N; i++)
rst = max(rst, max(D[getx(S[i].first)], E[gety(S[i].second)]));
cout << rst << endl;
return 0;
}
