#include <bits/stdc++.h>
using namespace std;
const double PI = acos(-1);
const double eps = 1e-9;
const int inf = 2000000000;
const long long infLL = 9000000000000000000;
inline bool checkBit(long long n, int i) { return n & (1LL << i); }
inline long long setBit(long long n, int i) { return n | (1LL << i); }
inline long long resetBit(long long n, int i) { return n & (~(1LL << i)); }
inline bool EQ(double a, double b) { return fabs(a - b) < 1e-9; }
inline bool isLeapYear(long long year) {
  return (year % 400 == 0) || (year % 4 == 0 && year % 100 != 0);
}
inline void normal(long long &a) {
  a %= 1000000007;
  (a < 0) && (a += 1000000007);
}
inline long long modMul(long long a, long long b) {
  a %= 1000000007, b %= 1000000007;
  normal(a), normal(b);
  return (a * b) % 1000000007;
}
inline long long modAdd(long long a, long long b) {
  a %= 1000000007, b %= 1000000007;
  normal(a), normal(b);
  return (a + b) % 1000000007;
}
inline long long modSub(long long a, long long b) {
  a %= 1000000007, b %= 1000000007;
  normal(a), normal(b);
  a -= b;
  normal(a);
  return a;
}
inline long long modPow(long long b, long long p) {
  long long r = 1;
  while (p) {
    if (p & 1) r = modMul(r, b);
    b = modMul(b, b);
    p >>= 1;
  }
  return r;
}
inline long long modInverse(long long a) {
  normal(a);
  return modPow(a, 1000000007 - 2);
}
inline long long modDiv(long long a, long long b) {
  return modMul(a, modInverse(b));
}
int dx[] = {0, 0, +1, -1};
int dy[] = {+1, -1, 0, 0};
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
int n, h, ex1[200005], ex2[200005], dist[200005];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  cin >> n >> h;
  for (int i = 1; i <= n; i++) {
    cin >> ex1[i] >> ex2[i];
  }
  for (int i = 2; i <= n; i++) {
    dist[i - 1] = ex1[i] - ex2[i - 1];
  }
  dist[n] = 1e9;
  int tmp = 0, tpp = 0, res, ans = -1;
  for (int i = 1, j = 1; i <= n; i++) {
    while (j <= n && tmp < h) {
      tmp += dist[j];
      tpp += ex2[j] - ex1[j];
      j++;
    }
    res = h + tpp;
    ans = max(ans, res);
    tmp -= dist[i];
    tpp -= ex2[i] - ex1[i];
  }
  cout << ans << '\n';
  return 0;
}
