#include <fstream>
#include <limits>
#include <algorithm>
#define INF numeric_limits<int>::max()
using namespace std;
ifstream in("orase.in");
ofstream out("orase.out");
pair<int,int> a[50001];
int n,m;
int main()
{
    in>>m>>n;
    for(int i=1;i<=n;i++)
        in>>a[i].first>>a[i].second;
    sort(a+1,a+n+1);
    int mx=-INF,sol=-INF;
    for(int i=1;i<=n;i++)
    {
        sol=max(sol,a[i].first+a[i].second+mx);
        mx=max(mx,a[i].second-a[i].first);
    }
    out<<sol<<'\n';
    return 0;
}