#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
int n,i,m,maxim,nr;
pair <int,int> v[50010];
int main(){
    fin>>m>>n;
    for(i=1;i<=n;i++){
        fin>>v[i].first>>v[i].second;
    }
    sort(v+1,v+n+1);
    for(i=1;i<=n;i++){
        if(v[i].first-v[i-1].first+v[i].second+maxim>nr){
            nr=v[i].first-v[i-1].first+v[i].second+maxim;
        }
        if(v[i].second<maxim+v[i].first-v[i-1].first){
            maxim=maxim+v[i].first-v[i-1].first;
        }
        else{
            maxim=v[i].second;
        }
    }
    fout<<nr<<"\n";

    return 0;
}