#include <bits/stdc++.h>
std::ifstream in("orase.in");
std::ofstream out("orase.out");
using namespace std;
//bool viz[50001];
vector<int>where;
int v[50001];


int main()
{
    int n , m ;
    in >> n >>m ;
    for(int i =1 ; i<= m ; ++i)
    {
        int x, y;
        in >> x >> y;
        if(!v[x]) {where.push_back(x);v[x]=y;}
        else {v[x]=max(v[x],y);}
    }
   sort(where.begin(),where.end());
   int t = 0;
 int dim = where.size();
for(int  i =0 ; i < dim-1 ; ++i)
    for(int j = i +1; j<=dim-1;++j)
      t = max(t,where[j]-where[i]+v[where[i]]+v[where[j]]);
out << t ;
    return 0;
}