#include <bits/stdc++.h>
using namespace std;
int n,c,i,ma,t,nr,mi,st,d,r,j,Ma;
pair<int,int>v[2010];
int main()
{
    ifstream f ("carnati.in");
    ofstream g ("carnati.out");
    f>>n>>c;
    for(i=1; i<=n; ++i)
        f>>v[i].first>>v[i].second;
    sort(v+1,v+n+1);
    for(i=1; i<=n; ++i)
    {
        nr=0;
        t=0;
        ma=0;
        for(j=1; j<=n; ++j)
        {
            if(v[j].second>=v[i].second)
            {
                if(t==0)t=v[j].first;
                ++nr;
            }
            else continue;
            if(ma<nr*v[i].second-(v[j].first-t+1)*c)ma=nr*v[i].second-(v[j].first-t+1)*c;
            else
            {
                --nr;
                break;
            }
        }
        Ma=max(Ma,ma);
    }
    g<<Ma<<'\n';
    return 0;
}