#include <iostream>
#include <fstream>
using namespace std;
ifstream in ("orase.in");
ofstream out ("orase.out");
const int N=1000000;
struct oras
{
    int d,l;
};

oras v[N];
bool cmp(oras x, oras y)
{
    return x.d<y.d;
}

int main()
{
    int i,n,m,dmax=-1000000000,dc=0,u=1;
    in>>m>>n;
    for(i=1;i<=n;i++)
    {
        in >> v[i].d >> v[i].l;
    }
    sort(&v[1],&v[n+1],cmp);
    for (i=2;i<=n;i++)
    {
        dc=v[i].l+v[u].l+v[i].d-v[u].d;
        if(dc>dmax) dmax=dc;
        if(v[i].l>v[u].l+(v[i].d-v[u].d))
        {
            u=i;
        }
    }
    out<<dmax;
    return 0;
}