#include <iostream>
#include <fstream>
using namespace std;
ifstream ka("orase.in");
ofstream ki("orase.out");
int m,n,d[50005],l[50005];
unsigned long long dif,maxim;
int abs(int t)
{
    if(t>=0)return t;
    return -t;
}
int main()
{
    ka>>m>>n;
    for(int i=1;i<=n;i++)
    {
        ka>>d[i]>>l[i];
    }
    for(int i=1;i<=n;i++)
    {
        for(int j=i+1;j<=n;j++)
        {
            dif=l[i]+abs(d[j]-d[i])+l[j];
            if(dif>maxim)
            maxim=dif;
        }
    }
    ki<<maxim;
}