#include <cstdio>
#include <algorithm>
#include <iostream>

using namespace std;
pair <int,int> v[50001];
int d[50001];
int main()
{
    FILE *fin=fopen ("orase.in","r");
    FILE *fout=fopen ("orase.out","w");
    int m,n,i,smax;
    fscanf (fin,"%d%d",&m,&n);
    for (i=1;i<=n;i++)
        fscanf (fin,"%d%d",&v[i].first,&v[i].second);
    // d[i] subsecv de s max care se termina pe poz i
    sort (v+1,v+n+1);
    smax=0;
    d[1]=v[1].second;
    for (i=1;i<=n;i++){
        smax=max(smax,d[i-1]+v[i].first-v[i-1].first+v[i].second);
        d[i]=max(v[i].second,d[i-1]+v[i].first-v[i-1].first);
    }
    fprintf (fout,"%d",smax);
    return 0;
}