#include <iostream>
#include <utility>
#include <vector>
#include <string>
#include <cmath>
#include <algorithm>
#include <functional>
#include <iomanip>
#include <map>
#include <set>
#include <queue>
using namespace std;


int main(int argc, char const *argv[])
{
  int N,D; cin >> N >> D;
  vector<pair<int,int>> task(N);
  for(auto &t : task)cin >> t.first >> t.second;
  vector<vector<int>> dp(N+1,vector<int>(2,-1));// 0 -> Tokyo 1 -> kyoto
  dp[0][0] = 0;
  auto chmax = [](int &x, int y) -> int
  {
    if(x < y)x = y; return x;
  };
  for(int i=0;i<N;++i)
  {
    if(dp[i][0] != -1)
    {
      chmax(dp[i+1][0],dp[i][0]+task[i].first);
      chmax(dp[i+1][1],dp[i][0] + task[i].second - D);
    }
    if(dp[i][1] != -1)
    {
      chmax(dp[i+1][1],dp[i][1]+task[i].second);
      chmax(dp[i+1][0],dp[i][1] + task[i].first - D);
    }
  }
  int answer = max(dp[N][0],dp[N][1]);
  std::cout << answer << std::endl;
}