#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin("carnati.in");
ofstream fout("carnati.out");
int n,i,c,x,pret,j;
struct numar{
    int t,p;
}v[100001];
int cmp(numar a,numar b){
    return a.t<b.t;
}
long long s,sMax;
int main()
{
    fin>>n>>c;
    sMax=-20000000000ll;
    for(i=1;i<=n;i++)
        fin>>v[i].t>>v[i].p;
    sort(v+1,v+n+1,cmp);
    for(i=1;i<=n;i++){
        x=v[i].p;
       if(v[1].p>=x)
           pret=x;
       else
          pret=0;
        s=pret-c;
        sMax=max(sMax,s);
        for(j=2;j<=n;j++){
            pret=x;
            if(v[j].p<x)
                pret=0;
            if(s+pret-(v[j].t-v[j-1].t)*c>pret-c)
                s=s+pret-(v[j].t-v[j-1].t)*c;
            else
                s=pret-c;
            sMax=max(s,sMax);
        }
    }
    fout<<sMax;
    return 0;
}