#include <bits/stdc++.h>
using namespace std;ifstream fin("carnati.in");ofstream fout("carnati.out");int n,c,a[2001],x;pair <int,int > v[2001];int main(){fin >> n >> c;int i,j;int rasp=-1;for(i=1;i<=n;i++){fin >> v[i].first >> v[i].second;}sort(v+1,v+n+1);v[0].first=v[1].first-1;for(i=1;i<=n;i++){for(j=1;j<=n;j++){a[j]=0;if(v[i].second<=v[j].second) a[j]=v[i].second;x=(v[j].first-v[j-1].first)*c;a[j]=max(a[j]+a[j-1]-x,a[j]-c);if(a[j]>rasp)rasp=a[j];}}fout << rasp;return 0;}