#include <iostream>
#include <fstream>

int d[50001],l[50001];

using namespace std;

int main()
{
    ifstream f("orase.in");
    ofstream g("orase.out");

    int n, i, m, j, smax=-2000000000, sc=-1;
    f>>m>>n;
    for(i=1;i<=n;i++)
        f>>d[i]>>l[i];
    for(i=1;i<=n;i++)
        for(j=i+1;j<=n;j++)
        {
            if(d[j]>d[i])
            sc=d[j]-d[i]+l[i]+l[j];
            else
            sc=d[i]-d[j]+l[i]+l[j];
            if(sc>smax)
                smax=sc;
        }
    g<<smax;
    return 0;
}