#include<fstream>
#include<iomanip>
using namespace std;

ifstream fin("orase.in");
ofstream fout("orase.out");

struct distanta{
       int d, l;         
};

distanta a[50002];

int m, n;

void Read();
void Solve();

int main()
{
    Read();
    Solve();
    
    fin.close();
    fout.close();
    
    return 0;
}

void Read()
{
     fin >> m >> n;
     for( int i = 1; i <= n; ++i )
          fin >> a[i].d >> a[i].l;
}

void Solve()
{
     int max_drum = -999;
     
     for( int i = 1; i < n; ++i )
          for( int j = i+1; j <= n; ++j )
               if( abs(a[j].d - a[i].d ) + a[i].l + a[j].l > max_drum )
                   max_drum = abs(a[j].d - a[i].d ) + a[i].l + a[j].l;
     
     fout << max_drum <<'\n';
}