#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; }