#include <cstdio>
#include <vector>
#include <algorithm>
#define d first
#define l second
using namespace std;
vector<pair<int,int> > v;
int n,m,i,maxi,sol,sc,d,l;
int main()
{
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);
    scanf("%d%d",&m,&n);
    for(i=1;i<=n;i++)
    {
        scanf("%d%d",&d,&l);
        v.push_back(make_pair(d,l));
    }
    sort(v.begin(),v.end());
    maxi=v[0].l-v[0].d;
    for(i=1;i<n;i++)
    {
        sc=maxi+v[i].d+v[i].l;
        if(maxi<v[i].l-v[i].d)
            maxi=v[i].l-v[i].d;
        if(sol<sc)
            sol=sc;
    }
    printf("%d\n",sol);
    return 0;
}