#include <fstream>
#include <iostream>
#include <algorithm>
#include <climits>

using namespace std;

ifstream f ("carnati.in");
ofstream g ("carnati.out");

const int NMAX = 1e6;
const long long VMAX = INT_MIN;



struct om{
  long long int pret;
  long long int timp;
} v[NMAX+1];

bool comp(om x, om y){
    return x.timp < y.timp;
}

int main()
{
    int n, angajat;
    f >> n  >> angajat;
    long long smax = VMAX;
    for(int i=1; i<=n; i++)
        f >> v[i].timp >> v[i].pret;
    sort(v, v+1+n, comp);
    long long sfinal = VMAX;
    for(int i=0; i<=n; i++){
        int pretSet = v[i].pret;
        int st = 0;
        long long smax = VMAX;
        long long sum = VMAX;
        for(int j=0; j<=n; j++){
            if(pretSet <= v[j].pret){
                if(sum - (long long)angajat*(1 + v[j].timp - v[st].timp) < 0)
                    sum = 0, st = j;
                sum += pretSet;
                smax = max(smax, sum - (long long)angajat*(1 + v[j].timp - v[st].timp));
            }
            
        }
        sfinal = max(sfinal, smax);
    }
    if(sfinal < 0)
        sfinal = 0;
    g<<sfinal;

    return 0;
}