#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");

long n, m, i, j, x, l, d, s, maxim;
pair <int, int> v[50002];
int main(){
    fin>>m>>n;
    for(i=1; i<=n; i++){
        fin>>v[i].first>>v[i].second;
    }
    sort(v+1, v+n+1);
    d=v[1].first;
    l=v[1].second;
    for(i=2; i<=n; i++){
        s=l+v[i].first+v[i].second-d;
        if(s>maxim)
            maxim=s;
        if(v[i].first-d+l<v[i].second){
            d=v[i].first;
            l=v[i].second;
        }
    }
    fout<<maxim;
}