#include <iostream>
#include <stdio.h>
#include <math.h>
#include <algorithm>
using namespace std;

struct pk{
	int time,gain;
};
int n,c;
pk a[2005];

void solve(){
	int localMax = 0;
	int newJ,max = -(1 << 30);
	bool entered = false;
	for(int i = 0; i < n; i++){
		newJ = 0;
		localMax = 0;
		entered = false;
		for(int j = 0; j < n; j++){
			if(a[i].gain <= a[j].gain){	
				entered = true;
				int g = a[i].gain - abs(a[j].time - a[newJ].time) * c;
				if(localMax + g >= a[i].gain)
					localMax += g;
				else
					localMax = a[i].gain;
				
				if(localMax > max)
					max = localMax;
				newJ = j;
			}
			else
				if(entered == false)
					newJ = j + 1;
		}
	}
	if(max > c)
		printf("%d\n",max - c);
	else
		printf("0\n");

}
int cmp(pk a, pk b)
{
    return a.time < b.time;
}
int main(){
	freopen("carnati.in","r",stdin);
	freopen("carnati.out","w",stdout);

	scanf("%d%d",&n,&c);
	int g,t;
	for(int i = 0; i < n; i++){
		scanf("%d%d",&t,&g);
		a[i].gain = g;
		a[i].time = t;
	}
	sort(a,a+n,cmp);
	solve();

	return 0;
}