#include <cstdio>
#include <algorithm>
using namespace std;
const int NMAX=2000;
int n,c,d[NMAX+5],answer;
struct Client {
    int t,p;
}v[NMAX+5];
bool Comp(Client a, Client b);
void Check(int number);
int main() {
    freopen("carnati.in","r",stdin);
    freopen("carnati.out","w",stdout);
    scanf("%d%d",&n,&c);
    v[0].t=-1500;
    for(int i=1;i<=n;++i)
        scanf("%d%d",&v[i].t,&v[i].p);
    sort(v+1,v+1+n,Comp);
    for(int i=1;i<=n;i++)
        Check(v[i].p);
    printf("%d",answer);
    return 0;}
void Check(int number) {
    for(int i=1;i<=n;i++) {
        if(v[i].p>=number)
            d[i]=max(number-c,d[i-1]+number-c*(v[i].t-v[i-1].t));
        else
            d[i]=max(-c,d[i-1]-c*(v[i].t-v[i-1].t));
        answer=max(d[i],answer);}}
bool Comp(Client a, Client b) {
    return a.t<b.t;}