#include <bits/stdc++.h>
using namespace std;
int n,c,i,ma,t,nr,mi;
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)
    {
        if(t==0)t=v[i].first;
        if(mi==0)mi=v[i].second;
        if(ma<min(mi,v[i].second)*(nr+1)-(v[i].first-t+1)*c)ma=min(mi,v[i].second)*(nr+1)-(v[i].first-t+1)*c,++nr,mi=min(mi,v[i].second);
        if(ma<v[i].second-c)
        {
            nr=1;
            mi=v[i].second;
            ma=v[i].second-c;
            t=v[i].first;
        }
    }
    g<<ma<<'\n';
    return 0;
}