using namespace std;
#include<iostream>
#include<fstream>
#include<algorithm>
#define oo 0x3f3f3f3f
struct nod{
int d;
int l;};
ofstream fout("orase.out");
nod a[50005];
int N,M;

bool cmp(nod i,nod j)
{
    return i.d<j.d;
}

void cit()
{int i;
    ifstream fin("orase.in");
    fin>>M>>N;
    for(i=1;i<=N;i++)
    {
     fin>>a[i].d>>a[i].l;


    }
    fin.close();

}

int main()
{int maxim,ans,i;

    cit();
    sort(a+1,a+N+1,cmp);
    maxim=-oo;
    ans=-oo;
    for(i=1;i<=N;i++)
    {

        ans=max(ans,maxim+a[i].l+a[i].d);
        maxim=max(maxim,a[i].l-a[i].d);
    }
    fout<<ans<<"\n";
    fout.close();

    return 0;
}