#include <bits/stdc++.h>
using namespace std;
vector<long long> graph[100005];
vector<pair<long long, long long> > graph_p[100005];
vector<pair<long long, long long> > op;
vector<long long> vis(100005, 0);
long long power(long long x, long long y) {
  long long res = 1;
  x = x % 1000000009;
  if (x == 0) return 0;
  while (y > 0) {
    if (y & 1) res = ((res % 1000000009) * (x % 1000000009)) % 1000000009;
    y = y >> 1;
    x = ((x % 1000000009) * (x % 1000000009)) % 1000000009;
  }
  return res;
}
void init_graph(long long n, long long m) {
  cin >> n >> m;
  for (long long i = 1; i <= m; i++) {
    long long a, b;
    cin >> a >> b;
    graph[a].push_back(b);
    graph[b].push_back(a);
  }
}
void sieve(long long n) {
  bool prime[n + 1];
  memset(prime, true, sizeof(prime));
  for (long long p = 2; p * p <= n; p++) {
    if (prime[p] == true) {
      for (long long i = p * p; i <= n; i += p) prime[i] = false;
    }
  }
}
long long n, h, l[2000005], r[2000005], ans;
long long sum[2000005], de[2000005];
void we_have_choices() {
  cin >> n >> h;
  for (long long i = 1; i <= n; i++) {
    cin >> l[i] >> r[i];
    sum[i] = sum[i - 1] + r[i] - l[i];
    if (i == 1)
      de[i] = 0;
    else
      de[i] = de[i - 1] + l[i] - r[i - 1];
  }
  for (long long i = 1; i <= n; i++) {
    long long pos = lower_bound(de + i, de + n + 1, de[i] + h) - de;
    ans = max(ans, sum[pos - 1] - sum[i - 1] + h);
  }
  cout << ans << "\n";
}
int32_t main() {
  ios_base::sync_with_stdio(false), cin.tie(nullptr);
  ;
  long long t = 1;
  while (t--) we_have_choices();
  return 0;
}
