#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
long long m,n,d[1100000],l[1100000],mx;
long long nat(long long a)
{
    if(a>0)
        return a;
    return -a;
}
int main()
{
    fin>>m>>n;
    for(int i=1;i<=n;i++)
        fin>>d[i]>>l[i];
    for(int i=2;i<=n;i++)
        for(int j=1;j<i;j++)
        {
            if(nat(d[i]-d[j])+l[i]+l[j]>mx)
                mx=nat(d[i]-d[j])+l[i]+l[j];
        }
    fout<<mx<<"\n";
    return 0;
}