#include <bits/stdc++.h>
using namespace std;
long long a[200010];
long long l[200010];
long long r[200010];
int main() {
  ios_base::sync_with_stdio(0);
  long long n, h;
  cin >> n >> h;
  vector<long long> rast2(n + 1, 0);
  for (int i = 1; i <= n; i++) {
    cin >> l[i] >> r[i];
    rast2[i] = r[i] - l[i];
  }
  for (int i = 1; i <= n; i++) {
    rast2[i] += rast2[i - 1];
  }
  vector<long long> rast(n + 1, 0);
  for (int i = 1; i < n; i++) {
    rast[i] = l[i + 1] - r[i];
  }
  rast[n] = 2e9;
  for (int i = 1; i <= n; i++) {
    rast[i] += rast[i - 1];
  }
  long long ans = 0;
  for (int i = 1; i <= n; i++) {
    long long r =
        lower_bound(rast.begin(), rast.end(), h + rast[i - 1]) - (rast.begin());
    if (r <= n) ans = max(ans, (rast2[r] - rast2[i - 1]) + h);
  }
  cout << ans;
}
