#include <fstream>
#include <algorithm>
#include <climits>
#define DIM 2001
using namespace std;
ifstream fin("carnati.in");
ofstream fout("carnati.out");
int n,c,k,sol,d[DIM];
struct om {
    int t,p;
}v[DIM],a[DIM];

bool cmp(om a,om b) {
    if (a.t==b.t)
        return a.p<=b.p;
    return a.t<b.t;
}

int main() {
    fin>>n>>c;
    for (int i=1;i<=n;i++)
        fin>>v[i].t>>v[i].p;
    sort(v+1,v+n+1,cmp);
    sol=INT_MIN;
    for (int i=1;i<=n;i++) {
        k=0;
        for (int j=1;j<=n;j++)
            if (v[j].p>=v[i].p)
                a[++k]=v[j];
        d[1]=v[i].p-c;
        sol=max(sol,d[1]);
        for (int j=2;j<=k;j++) {
            d[j]=max(d[j-1]+v[i].p-c*(a[j].t-a[j-1].t),v[i].p-c);
            sol=max(sol,d[j]);
        }
    }
    fout<<sol;
    return 0;
}