#include <fstream>
#include <algorithm>
using namespace std;
#define NMax 50001
#define FOR(i,a,b) for ( int i = a; i <= b; ++i )

ifstream in ( "orase.in" );
ofstream out ( "orase.out" );

struct oras
{
	int d, l;
} v [ NMax ];
int N, M;

bool comp ( oras a, oras b )
{
	return a .d < b .d;
}

int main ()
{
	in >> M >> N;
	
	FOR ( i, 1, N )
		in >> v [ i ] .d >> v [ i ] .l;
	
	sort ( v + 1, v + N + 1, comp );
	
	int i0 = 1, dmax = 0, i1 = 1;
	FOR ( i, 2, N )
		if ( v [ i ] .l + v [ i0 ] .l + v [ i ] .d - v [ i0 ] .d > dmax )
		{
			dmax = v [ i ] .l + v [ i0 ] .l + v [ i ] .d - v [ i0 ] .d;
			i1 = i;
		}
	
	i0 = i1;
	FOR ( i, i0, N )
		if ( v [ i ] .l + v [ i0 ] .l + v [ i ] .d - v [ i0 ] .d > dmax )
			dmax = v [ i ] .l + v [ i0 ] .l + v [ i ] .d - v [ i0 ] .d;
	for ( int i = i0; i >= 1; --i )
	{
		if ( v [ i0 ] .l + v [ i0 ] .d - v [ i ] .d + v [ i ] .l > dmax )
			dmax = v [ i0 ] .l + v [ i0 ] .d - v [ i ] .d + v [ i ] .l;
	}
	
	out << dmax;
	
	return 0;
}