#include <iostream>
#include <cstdio>
#include <algorithm>

#define NMax 50005
#define x first
#define y second

using namespace std;

pair <int, int> City[NMax];
int N, L, DP[NMax], SDist;

inline int Max (int a, int b)
{
    if (a>b)
    {
        return a;
    }
    return b;
}

inline int Dist (int i, int j)
{
    return City[j].x-City[i].x+City[j].y+City[i].y;
}

void Read ()
{
    freopen ("orase.in", "r", stdin);
    scanf ("%d %d", &L, &N);
    for (int i=1; i<=N; ++i)
    {
        scanf ("%d %d", &City[i].x, &City[i].y);
    }
}

void Print ()
{
    freopen ("orase.out", "w", stdout);
    printf ("%d\n", SDist);
}

int main()
{
    Read ();
    sort (City+1, City+N+1);
    DP[2]=1;
    for (int i=3; i<=N; ++i)
    {
        int D1=Dist (DP[i-1], i);
        int D2=Dist (i-1, i);
        if (D1>D2)
        {
            DP[i]=DP[i-1];
        }
        else
        {
            DP[i]=i-1;
        }
        SDist=Max (SDist, Dist (DP[i], i));
    }
    Print ();
    return 0;
}