#include <fstream>
#import <algorithm>
#import <vector>
#import <map>
#import <deque>
#import <cassert>
#import <cmath>
using namespace std;
ifstream cin("carnati.in");
ofstream cout("carnati.out");
int
main()
{
    int c,n;
    cin>>n>>c;
    vector<pair<int,int>>a(n);
    for(auto &c:a)cin>>c.first>>c.second;
    sort(a.begin(),a.end());
    int rez=-2e9;
    for(int i=0;i<n;i++)
    {
        int s=-2e9,start=0;
        for(int j=0;j<n;j++)
        {
            if(a[j].second>=a[i].second)
            {
                s+=a[i].second;
                if(s-(a[j].first-start+1)*c<=a[i].second-c)
                {
                    s=a[i].second;
                    start=a[j].first;
                }
                rez=max(rez,s-(a[j].first-start+1)*c);
                if(i==2 && s-(a[j].first-start+1)*c==381)cout<<s;
            }
        }
    }
    cout<<rez;
}