#include <cstdio>
#include <algorithm>
using namespace std;

#define Nmax 50002
#define inf 0x3f3f3f3f

FILE *f = fopen ( "orase.in", "r" );
FILE *g = fopen ( "orase.out", "w" );

pair < int, int > v[Nmax];

int main(){

    int N, M, poz, maxdif = -inf, sol = -inf;

    fscanf ( f, "%d%d", &N, &M );

    for ( int i = 1; i <= N; ++i )
        fscanf ( f, "%d%d", &v[i].first, &v[i].second );

    sort ( v + 1, v + N + 1 );

    for ( int i = 1; i <= N; ++i ){

        sol = max ( sol, v[i].first + v[i].second + maxdif );

        if ( v[i].second - v[i].first > maxdif )
            maxdif = v[i].second - v[i].first;
    }

    fprintf ( g, "%d", sol );

    return 0;
}