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#include <unistd.h>
#include <string>
#include <vector>
#include <map>
#include <set>
#include <utility>
#include <deque>
#include <iostream>
#include <algorithm>
#include <limits>
#include <cmath>

// using namespace std;
#define REP(i,n) for(int _n=(n), i=0;i<_n;++i)
#define FOR(i,a,b) for(int i=(a),_b=(b);i<=_b;++i)
#define FORD(i,a,b) for(int i=(a),_b=(b);i>=_b;--i)
#define TRACE(x) cerr << "TRACE(" #x ")" << endl;
#define DEBUG(x) cerr << #x << " = " << (x) << endl;
typedef long long LL;
typedef unsigned long long ULL;
using VINT = std::vector<int>;
using VLL = std::vector<LL>;
using VULL = std::vector<ULL>;

struct Node {
  long hight = 0;
  int color = 0;  
  
  bool operator< (const Node& other) const {
    if (hight == other.hight) return color < other.color;
    return hight < other.hight;
  }
};


int main() {
  std::ios_base::sync_with_stdio(false);
  int n, c, a, w;

  std::cin >> n >> c;


  // std::vector<Node> colorToHight;
  // colorToHight.resize(500000);

  std::set<Node> uniqNodes;

  REP(i, n) {
    std::cin >> a >> w;
    uniqNodes.insert(Node{a, w});
  }

  std::vector<Node> nodes(uniqNodes.rbegin(), uniqNodes.rend());
  
  std::map<int, Node> colorToHight;
  for (auto& node : nodes) {
    if (colorToHight.find(node.color) == colorToHight.end()) {
      colorToHight[node.color] = node;
      continue;
    }
    
    long localMax = 0l;

    for (auto& m : colorToHight) {
      if (m.second.hight == node.hight) continue;
      if (m.first == node.color)
        localMax = std::max(localMax, m.second.hight + node.hight);
      else if (node.hight > c)
        localMax = std::max(localMax, m.second.hight + node.hight - c);
    
        
    }

    if (colorToHight[node.color].hight < localMax) {
      colorToHight[node.color] = Node{localMax, node.color};
    }
  }
  
  long max = 0l;

  for (auto& m : colorToHight) {
    max = std::max(max, m.second.hight);
  }

  std::cout << max << std::endl;
  
  return 0;
}