#include <fstream>
#include <algorithm>

#define inf 3000001

using namespace std;

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

struct street
{
    int l,d;
}v[50001];

int n,m,maxv,maxd;

bool cmp (street a, street b)
{
    return (a.d < b.d);
}

int main()
{
    fin>>m>>n;

    for (int i=1; i<=n; ++i)
    {
        fin>>v[i].d>>v[i].l;
    }

    sort (v+1,v+n+1,cmp);

    maxv = -inf, maxd = -inf;

    maxd = v[1].l;

    for (int i=2; i<=n; ++i)
    {
        maxd += v[i].d - v[i-1].d;

        maxv = max (maxv,maxd+v[i].l);
        maxd = max (maxd,v[i].l);
    }

    fout<<maxv;
}