#include <stdio.h>
#include <stdlib.h>
#define N 2010
int t[N],p[N],best[N],n,c,max=0;
int maxim(int a,int b){
	if (a>b)
      return a;
    return b;
}
void scan(){
	int i;
	freopen("carnati.in","r",stdin);
    freopen("carnati.out","w",stdout);
    scanf("%d%d",&n,&c);
    for (i=1;i<=n;++i)
       scanf("%d%d",&t[i],&p[i]);
}
void swap(int i,int j){
	int aux;
    aux=t[i];
    t[i]=t[j];
    t[j]=aux;
    aux=p[i];
    p[i]=p[j];
    p[j]=aux;
}
void down_heap(int i,int n){
	int max=i;
    if (2*i<=n && t[max]<t[2*i])
      max=2*i;
    if (2*i+1<=n && t[max]<t[2*i+1])
      max=2*i+1;
    if (max!=i){
      swap(i,max);
      down_heap(max,n);
    }
}
void sort(){
	int i;
    for (i=n/2;i>=1;--i)
       down_heap(i,n);
    for (i=n;i>1;--i){
       swap(1,i);
       down_heap(1,i-1);
    }
}
void solve(){
	int i,j;
    t[0]=p[0]=-10;
    for (i=1;i<=n;++i)
       for (j=1;j<=n;++j){
          	if (p[j]>=p[i])
              best[j]=maxim(best[j-1]-(t[j]-t[j-1])*c+p[i],p[i]-c);
            else
              best[j]=maxim(best[j-1]-(t[j]-t[j-1])*c,-c);
            if (best[j]>max)
              max=best[j];
            best[j-1]=0;
       }
    printf("%d\n",max);
}
void print(){
	fclose(stdin);
    fclose(stdout);
    exit(0);
}
int main(){
     scan();
     sort();
     solve();
     print();
}