#include <iostream>
#include <fstream>
#include <vector>
#include <algorithm>
#define nmax 50999
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
vector< pair<int,int> > v;

bool cmp(pair<int,int> a,pair<int,int> b)
{if(a.first<b.first)return 1;return 0;}

int main()
{int n,m,i,x,y;
fin>>n>>m;
    for(i=1;i<=m;i++)
        {fin>>x>>y;
         v.push_back(make_pair(x,y));
        }
    sort(v.begin(),v.end(),cmp);
     int maxi=v[0].second-v[0].first;
    int rez=0x80000000;
int j;
    for(j=1;j<m;j++)
        {rez=max(rez,  v[j].first+v[j].second+maxi);
         maxi=max(maxi,v[j].second-v[j].first);
        }
    fout<<rez;
}