#include <fstream>
#include <algorithm>
#include <vector>
using namespace std;
int main()
{
    ifstream fin("orase.in");
    ofstream fout("orase.out");
    vector< pair<int,int> > V;
    int N,M,i,j,sol=0; 
    fin>>M>>N;
    while (N--)
    {
          fin>>i>>j;
          V.push_back(make_pair(i,j));
    }
    sort(V.begin(),V.end());
    j=0;
    for (i=1;i<(int)V.size();++i)
    {
        sol=max(sol,V[i].second+V[j].second+V[i].first-V[j].first);
        if (V[j].second+V[i].first-V[j].first < V[i].second) j=i;
    }  
    fout<<sol;
    return 0;
}