#include<stdio.h>
long max-32000,t1,t2,j,i,S,ct,n,c,tmax,smax;
long t[2001],s[2001];
int main(void)
{
freopen("carnati.in","r",stdin);
freopen("carnati.out","w",stdout);
scanf("%ld%ld",&n,&c);
for(i=1;i<=n;i++)
	{
	scanf("%ld%ld",&t[i],&s[i]);
	if(t[i]>tmax)
	tmax=t[i];
	if(s[i]>smax)
	smax=s[i];
	}
for(t1=1;t1<=tmax;t1++)
for(t2=t1;t2<=tmax;t2++)
	{
	for(j=1;j<=n;j++)
		{
		ct=s[j];
		for(i=1;i<=n;i++)
			if(t[i]>=t1&&t[i]<=t2&&ct<=s[i])
		S+=ct;
		S=S-(c*(t2-t1+1));
		if(S>max)
		max=S;
		}
	}
printf("%ld\n",max);
return 0;
}