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

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

const int N=50000;
struct  oras
{
    int d,l;
};

bool cmp(oras x, oras y)
{
    return(x.d<y.d);
}

int dist(oras x, oras y)
{
    return (x.l+y.l+y.d-x.d);
}

int main()
{
    int n,m,dmax,dc;
    oras v[N];
    fin>>m>>n;
    for(int i=0; i<n; i++)
    {
        fin>>v[i].d>>v[i].l;
    }
    fin.close();
    sort(v,v+n,cmp);
    dc=dmax=dist(v[0],v[1]);
    int u=0;
    for(int i=2; i<n; i++)
    {
        if(dist(v[i-1],v[i])>dist(v[u],v[i]))
        {
            u=i-1;
        }
        dc=dist(v[u],v[i]);
        if(dc>dmax)
        {
            dmax=dc;
        }
    }
    fout<<dmax;
    fout.close();
    return 0;
}