#include <iostream>

 #include <cstdio>

 #include <vector>

 #include <algorithm>

 using namespace std;

 typedef long long ll;

 const ll INF = 0x3f3f3f3f;

 int main()

 {

     int n, m;

     while (scanf("%d %d", &n, &m) != EOF)

     {

         vector<pair<int, int>> v;

         int x, y;

         for (int i = ; i < n; i++)

         {

             scanf("%d %d", &x, &y);

             v.push_back(pair<int, int>(x, y));

         }

         sort(v.begin(), v.end());

         vector<pair<int, int>> res;

         res.push_back(v[]);

         for (int i = ; i < v.size(); i++)

         {

             if (v[i].first > res.back().second + ) res.push_back(v[i]);

             else res.back().second = max(res.back().second, v[i].second);

         }

         vector<ll> sum(res.size() - );

         for (int i = ; i < res.size() - ; i++)

             sum[i] = (ll)res[i + ].first -  - res[i].second;

         sum.push_back(INF);

         for (int i = ; i < sum.size(); i++)

             sum[i] += sum[i - ];

         ll maxn = ;

         for (int i = ; i < sum.size(); i++)

         {

             ll tmp = (i ==  ?  : sum[i - ]);

             int pos = upper_bound(sum.begin() + i, sum.end(), m + tmp) - sum.begin();

             ll tmp2 = (pos == i ? m : m - (sum[pos - ] - tmp));

             maxn = max(maxn, res[pos].second - res[i].first +  + tmp2);

         }

         printf("%lld\n", maxn);

     }

     return ;

 }