import bisect
import sys, os, io
input = io.BytesIO(os.read(0, os.fstat(0).st_size)).readline
n, h = map(int, input().split())
u, v = [], []
for _ in range(n):
x1, x2 = map(int, input().split())
u.append(x1)
v.append(x2)
s = [0]
for i in range(n - 1):
s.append(s[-1] + u[i + 1] - v[i])
la = v[-1]
ls = s[-1]
ans = 0
for i in range(n):
x1, x2, si = u[i], v[i], s[i]
if ls - si < h:
ans = max(ans, la + h + si - ls - x1)
break
j = bisect.bisect_left(s, si + h) - 1
ans = max(ans, v[j] + h + si - s[j] - x1)
print(ans)#include <bits/stdc++.h>
//#include "debug.hpp"
using namespace std;
#define ll long long
#define tc ll tc;cin>>tc ;while(tc--)
#define li cout<<'\n';
#define simba iostream::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define in(arr, n) for(int i = 0 ; i<(n) ; i++) {cin>>(arr)[i];}
#define out(arr, n) for(int i = 0 ; i<(n) ; i++) {cout<<(arr)[i]<<" ";} cout<<"\n";
#define sz(x) ((int)(x.size()))
#define rall(v) rbegin(v), rend(v)
#define all(x) x.begin(),x.end()
#define yes cout << "Yes" << endl;
#define no cout << "No" << endl;
const int N = 1e5 + 5, INF = INT_MAX, MOD = 1e9 + 7,LOG = 20;
int Xaxis[8]={0,0,-1,1,-1,-1,1,1}; //right,left,up,down
int Yaxis[8]={1,-1,0,0,-1,1,1,-1}; //right,left,up,down
///vector input
template<class T>
istream &operator>>(istream &is, vector<T> &v) {
for (auto &i: v)
is >> i;
return is;
}
//// بسم الله الرحمن الرحيم
void solve() {
int n, h;
cin >> n >> h;
vector<pair<int, int>> v(n);
for (int i = 0; i < n; ++i) {
cin >> v[i].first >> v[i].second;
}
v.push_back({2e9, 2e9}), n++;
ll l = 0, r = 0, ans = 0, sum = 0;
while (r < n) {
while (v[r].first - v[l].first - sum >= h) {
sum -= (v[l].second - v[l].first);
l++;
}
sum += v[r].second - v[r].first;
ans = max(ans, sum), r++;
}
cout << ans + h;
}
int main() {
simba
// tc{
solve();
// }
return 0;
}
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