#include<cstdio>
#include<algorithm>

struct Client
{
    int timp;
    int pret;
};

Client v[2000];
bool cmp(Client a,Client b)
{
    return a.timp<b.timp;
}

int main()
{
    int n,k,i,s,maxp=0,j,q;
    FILE*fi,*fo;
    fi=fopen("carnati.in","r");
    fo=fopen("carnati.out","w");
    fscanf(fi,"%d%d",&n,&k);
    for(i=0; i<n; i++)
        fscanf(fi,"%d%d",&v[i].timp,&v[i].pret);
    std::sort(v,v+n,cmp);
    maxp=0;
    for(i=0; i<n; i++)
    {
        s=-k;
        for(j=0; j<n; j++)
        {
            q=0;
            if(v[j].pret>=v[i].pret)
            {
                q=v[i].pret;
            }
            if(s-(v[j].timp-v[j-1].timp-1)*k<0)
            {
                s=q-k;
            }
            else
            {
                s=s+q-(v[j].timp-v[j-1].timp)*k;
            }
            if(s>maxp)
                maxp=s;
        }
    }
    fprintf(fo,"%d",maxp);
    fclose(fi);
    fclose(fo);
    return 0;
}