#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];

int main()
{
    f>>M>>N;
    for(int i=1; i<=N; i++)
    {
        int x,y;
        f>>x>>y;
        dist[i].Di = x;
        dist[i].Li = y;
    }
    for(int i=1; i<=N; i++)
        for(int j=1;j<=N;j++)
        {
            int var = max(dist[i].Di,dist[j].Di)-min(dist[i].Di,dist[j].Di) + dist[i].Li + dist[j].Li;
            if(var > Max)
                Max = var;
        }

    g<<Max;
    return 0;
}