#include <bits/stdc++.h>
using namespace std;
int n,c,i,ma,t,nr,mi,st,d,r,j,Ma,mma,tt,nrr,ans[2010];
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)
    {
        for(j=1; j<=n; ++j)
        {
            ans[j]=0;
            if(v[i].second<=v[j].second)ans[j]=max(ans[j-1]-(v[j].first-v[j-1].first)*c+v[i].second,v[i].second-c);
            else ans[j]=ans[j-1]-(v[j].first-v[j-1].first)*c;
            Ma=max(Ma,ans[j]);
        }
    }
    g<<Ma<<'\n';
    return 0;
}