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#include <cstdio>
#include <vector>
#include <map>
#include <set>

using namespace std;

typedef long long LL;

int main() {
  int n, c; scanf(" %d %d", &n, &c);  
  
  map<int, LL> prev;
  set<pair<LL, int>, greater<> > prevR;
  map<int, LL> curr;

  int lastA = 0;
  for (int i=0; i<n; ++i) {
    int a, w; scanf(" %d %d", &a, &w);
    if (a > lastA) {
      lastA = a;
      
      for (const auto& el: curr) {
        auto it = prev.find(el.first);
        if (it == prev.end()) {
          prev.insert(el);
          prevR.insert(make_pair(el.second, el.first));
        } else {
          prevR.erase(make_pair(it->second, it->first));
          it->second = max(it->second, el.second);
          prevR.insert(make_pair(it->second, it->first));
        }
      }
      curr = map<int, LL>();
    }

    LL res = a;
    // printf("Maybe %d\n", a);
    if (prev.contains(w)) {
      // printf("Maybe also %lld\n", prev[w] + a);
      res = max(res, prev[w] + a);
    }
    if (!prevR.empty()) {
      // printf("Maybe but %lld\n", prevR.begin()->first + a - c);
      res = max(res,  prevR.begin()->first + a - c);
    }

    // printf("After %d computed %d -> %lld\n", a, w, res);

    auto it = curr.find(w);
    if (it != curr.end()) {
      it->second = max(it->second, res);
    } else {
      curr[w] = res;
    }
  }

  LL res = 0;
  for (const auto& el: prev) {
    res = max(res, el.second);
  }
  for (const auto& el: curr) {
    res = max(res, el.second);
  }
  printf("%lld\n", res);
  return 0;
}