#include <fstream>
#include <algorithm>
using namespace std;

ifstream fin("orase.in");
ofstream fout("orase.out");

int n, m, i, 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);
    s=v[n].second;
    for(i=n-1;i>=1;i--){
        s+= v[i + 1].first - v[i].first ;
        maxim=max( maxim ,s + v[i].second );
        s=max(s,v[i].second) ;
    }

    fout<<maxim;
}