#include <iostream>
#include <fstream>

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

const int nmax=50005;
const int INF=100000000;
struct val{
    int d,l;
};
val v[nmax];

int cmp(val x,val y)
{
    return(x.d<y.d);
}

int main()
{
    int m,n;
    fin>>m>>n;
    for(int i=1;i<=n;i++)
    {
        fin>>v[i].d>>v[i].l;
    }
    long long ansewer=-INF,min_dif=0;
    for(int i=1;i<=n;i++)
    {

        long long  candidat=min_dif-(v[i].d-v[i].l);
        if(candidat>ansewer)
            ansewer=candidat;

        if(v[i].d+v[i].l>min_dif)
        {
           min_dif=v[i].d+v[i].l;
        }
    }
    fout<<ansewer;
    return 0;
}