#include <fstream> 
#include <algorithm> 
#include <vector> 
#define MAX 50001 
using namespace std; 

long l[MAX], d[MAX]; 
long m, n, i, j, x, y; 

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

int main() 
{ 
    fin >> m >> n; 
    for ( i = 0; i < n; ++i) 
    { 
        fin >> x >> y; 
        d[i] = x * 100000 + y; 
    }     
    sort(d, d + n); 
    for ( i = 0; i < n; i++) 
    { 
        l[i] = d[i] % 100000; 
        d[i] = d[i] / 100000; 
    }    
    long dt = l[0] - d[0]; 
    long  maxt  = 0, dn, max; 

     for ( i = 1; i < n; ++i) 
     { 
          max = 0; 
          dn = l[i] - d[i]; 
          max = l[i] + d[i] + dt; 
          if ( dn > dt) 
                    dt = dn; 
          if(maxt < max) 
                    maxt = max; 
     } 

     fout << maxt << '\n'; 
     fin.close(); 
     fout.close(); 
     return 0; 
}