#include<stdio.h>
#include<algorithm>
#define x first
#define y second
using namespace std ;
pair < long , long > a [ 50007 ] ;
inline bool cmp ( pair < long , long > a , pair < long , long > b )
{
    return a . x < b . x ;
}
long n , m , max2 ;
int main ( )
{
    freopen ( "orase.in" , "r" , stdin ) ;
    freopen ( "orase.out" , "w" , stdout ) ;
    scanf ( "%ld %ld" , & m , & n ) ;
    for ( long i = 1 ; i <= n ; ++ i )
        scanf ( "%ld %ld" , & a [ i ] . x , & a [ i ] . y ) ;
    sort ( a + 1 , a + n + 1 , cmp ) ;
    long u = 0 ;
    for ( long i = 1 ; i <= n ; ++ i )
    {
        max2 = max ( max2 , a [ u ] . y + a [ i ] . y + a [ i ] . x - a [ u ] . x ) ;
        if ( a [ i ] . x - a [ u ] . x + a [ u ] . y < a [ i ] . y )
            u = i ;
    }
    printf ( "%ld" , max2 ) ;
    return 0 ;
}