#include <iostream>
#include <fstream>
                                /// Basic n*n
using namespace std;
ifstream f ("orase.in");
ofstream g ("orase.out");

int N,M;
int Max;

struct d{
    int Di;
    int Li;
}dist[50010],dis;
/*
dist[i].Di = x;
        dist[i].Li = y;
*/
int main()
{
    f>>M>>N;
    int furthest = 0;
    for(int i=1; i<=N; i++)
    {
        int x,y;
        f>>x>>y;
        dist[i].Di = x;
        dist[i].Li = y;
        if (x + y > furthest)
        {
            furthest = x + y;
            dis.Di = x;
            dis.Li = y;
        }

    }
    for(int i=1;i<=N;i++)
    {
        int var = max(dist[i].Di,dis.Di)-min(dist[i].Di,dis.Di) + dist[i].Li + dis.Li;
        if(var > Max)
                Max = var;
    }
    g<<Max;
    return 0;
}