#include <iostream>
#include <stdio.h>
#include <algorithm>

using namespace std;

int n,c,i,sum,p,hmax,x,j;

struct carnati{
    int t,p;
}v[2003];

int cmp(carnati a,carnati b){
    return (a.t<b.t);
}

int main()
{
    freopen("carnati.in","r",stdin);
    freopen("carnati.out","w",stdout);

    scanf("%d%d",&n,&c);

    for(i=1;i<=n;i++)
        scanf("%d%d",&v[i].t,&v[i].p);

    sort(v+1,v+n+1,cmp);

    v[0].t=v[1].t;
    for(i=1;i<=n;i++){
        sum=0;
        p=v[i].p;

        for(j=1;j<=n;j++){
            x=sum-(v[j].t-v[j-1].t)*c;

            if(x<0){
                sum=0;
            }else{
                sum=x;
            }


            if(v[j].p>=p)
                sum=sum+p;

            hmax=max(hmax,sum);

        }

    }

    printf("%d",hmax-c);


    return 0;
}