#include<bits/stdc++.h>
using namespace std;
#define ll long long
inline int read(){
   int s=0,w=1;
   char ch=getchar();
   while(ch<'0'||ch>'9'){if(ch=='-')w=-1;ch=getchar();}
   while(ch>='0'&&ch<='9') s=s*10+ch-'0',ch=getchar();
   return s*w;
}
pair<int,int> a[1<<20];
ll b[1<<20],f[1<<20],g,ans;
signed main()
{
	int n=read(),m=read();
	for(int i=1; i<=n; ++i)
		a[i].first=read(),a[i].second=read();
	sort(a+1,a+n+1),reverse(a+1,a+n+1);
	for(int i=1,j=1; i<=n; ++i)
	{
		while(a[j].first>a[i].first)
			f[a[j].second]=max(f[a[j].second],b[j]),
			g=max(g,b[j++]);
		ans=max(ans,b[i]=a[i].first+max(g-m,f[a[i].second]));
	}
	printf("%lld\n",ans);
	return 0;
}