#include<stdio.h>
#include<vector>
#include<algorithm>
using namespace std;
//#define NMAX 50002

vector< pair< int,int >  > V;

int main()
{
	freopen("orase.in","r",stdin);
	freopen("orase.out","w",stdout);

	int N,M;
	scanf("%d%d",&N,&M);

	int i,a1,a2;
	for(i=1; i<=N; ++i)
	{
		scanf("%d%d\n",&a1,&a2);
		V.push_back( make_pair(a1,a2) );
	}

	sort( V.begin(), V.end() );

	int p1;
	int q2=V[0].first,c2=V[0].second;
	int ANS=-1;
	for(p1=1; p1<N; ++p1)
	{
		a1= V[ p1 ].first - q2 + V[ p1 ].second + c2;
		if( a1>ANS ) ANS=a1;

		if( V[ p1 ].second > a1- V[ p1 ].second )
			q2=V[ p1 ].first,c2=V[ p1 ].second;
	}
	
	printf("%d\n",ANS);

	return 0;
}