#include <fstream>
#include <algorithm>
#define x first
#define y second
using namespace std;
pair <int, int> v[50001];
int d[50001],i,n,m,sol;
ifstream fin ("orase.in");
ofstream fout ("orase.out");

int main (){

    fin>>m>>n;
    for (i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    sort (v+1,v+n+1);
    d[1] = v[1].y;
    for (i=2;i<=n;i++){
        sol = max(sol,d[i-1] + (v[i].x - v[i-1].x) + v[i].y);
        d[i] = max (d[i-1] + (v[i].x - v[i-1].x), v[i].y);
    }
    fout<<sol;
    return 0;
}